Reduce toolchain size by preserving TensorFlow library symlinks.

Bazel produces multiple TensorFlow library artifacts:
```
$ ls -alh tensorflow/bazel-bin/tensorflow
 18B libtensorflow.so -> libtensorflow.so.1
 23B libtensorflow.so.1 -> libtensorflow.so.1.14.0
277M libtensorflow.so.1.14.0
```

Previously, TensorFlow libraries were copied via `cp -p`.
This did not preserve symlinks, leading to library duplication:
```
$ ls -alh <TOOLCHAIN_BEFORE>.xctoolchain/usr/lib/swift/macosx
 18B libtensorflow.so -> libtensorflow.so.1
277M libtensorflow.so.1
277M libtensorflow.so.1.14.0 # duplicate library
```

Now, `cp -a` is used, which preserves symlinks:
```
$ ls -alh <TOOLCHAIN_AFTER>.xctoolchain/usr/lib/swift/macosx
 18B libtensorflow.so -> libtensorflow.so.1
277M libtensorflow.so.1
```

This combined with removing libtensorflow_framework.so dependency
(swiftlang#27029) led to a macOS toolchain
size reduction from 3.99 GB to 3.62 GB on the tensorflow-0.5 branch.

Author: dan-zheng

stdlib/public/CTensorFlow/CMakeLists.txt

@@ -98,17 +98,6 @@ foreach(sdk ${TARGET_SDKS})
       COMPONENT stdlib
       )
   endforeach()
-
-  # Install TensorFlow libraries in each SDK subdirectory.
-  # FIXME: Currently, this installs the same version of TensorFlow libraries
-  # everywhere.
-  # To properly support multiple targets, TF library/include variables should be
-  # SDK/architecture-specific, just like the ones for ICU.
-  swift_install_in_component(
-    FILES "${TF_LIBRARIES}"
-    DESTINATION "lib/swift/${SWIFT_SDK_${sdk}_LIB_SUBDIR}"
-    COMPONENT stdlib
-    )
 endforeach()
 add_custom_target(ctensorflow_modulemap DEPENDS ${ctensorflow_modulemap_target_list})
 

utils/build-script-impl

@@ -2583,15 +2583,18 @@ for host in "${ALL_HOSTS[@]}"; do
                     TF_LIB_DIR="${SWIFT_LIB_PATH}/swift/${SWIFT_HOST_VARIANT}"
                     call mkdir -p "${TF_LIB_DIR}"
 
-                    # Bazel-built libraries do not have write permission, which is
-                    # problematic for overwriting/stripping symbols. Thus, write
-                    # permission is added here.
                     for lib_name in tensorflow; do
-                        lib=".*lib${lib_name}.so[0-9.]*"
-                        dylib=".*lib${lib_name}[0-9.]*.dylib"
-                        find "${TF_LIB_DIR}" \( -regex "${lib}" -o -regex "${dylib}" \) -exec rm -f {} \;
-                        find "${TENSORFLOW_HOST_LIB_DIR}" \( -regex "${lib}" -o -regex "${dylib}" \) -exec chmod +w {} +
-                        find "${TENSORFLOW_HOST_LIB_DIR}" \( -regex "${lib}" -o -regex "${dylib}" \) -exec cp -p {} "${TF_LIB_DIR}" \;
+                        lib_full_name="lib${lib_name}.so"
+                        # Wipe existing TensorFlow libraries in the destination
+                        # directory. This ensures that old versions of libraries
+                        # are not copied, taking up unnecessary space.
+                        call rm -rf "${TF_LIB_DIR}/${lib_full_name}"*
+                        # Copy TensorFlow libraries, preserving symlinks.
+                        call find "${TENSORFLOW_HOST_LIB_DIR}" \( -regex ".*${lib_full_name}[0-9.]*" \) -exec cp -a -v {} "${TF_LIB_DIR}" \;
+                        # Bazel-built libraries do not have write permission, which is
+                        # problematic for overwriting/stripping symbols. Thus, write
+                        # permission is added here.
+                        call chmod +w "${TF_LIB_DIR}/${lib_full_name}"*
                     done
 
                     cmake_options=(
@@ -4008,15 +4011,17 @@ for host in "${ALL_HOSTS[@]}"; do
                     exit 1
                 fi
                 echo "--- Installing ${product} ---"
-                TF_BUILD_DIR="$(build_directory ${host} ${product})"
-                TF_LIBDIR="$(build_directory ${host} swift)/lib/swift/${SWIFT_HOST_VARIANT}"
+                TF_LIB_DIR="$(build_directory ${host} swift)/lib/swift/${SWIFT_HOST_VARIANT}"
                 TF_DEST_DIR="$(get_host_install_destdir ${host})$(get_host_install_prefix ${host})lib/swift/${SWIFT_HOST_VARIANT}"
                 mkdir -p "${TF_DEST_DIR}"
                 for lib_name in tensorflow; do
-                    lib=".*lib${lib_name}.so[0-9.]*"
-                    dylib=".*lib${lib_name}[0-9.]*.dylib"
-                    find "${TF_LIBDIR}" \( -regex "${lib}" -o -regex "${dylib}" \) -exec echo "{} => ${TF_DEST_DIR}" \;
-                    find "${TF_LIBDIR}" \( -regex "${lib}" -o -regex "${dylib}" \) -exec cp -a {} "${TF_DEST_DIR}" \;
+                    lib_full_name="lib${lib_name}.so"
+                    # Wipe existing TensorFlow libraries in the destination
+                    # directory. This ensures that old versions of libraries
+                    # are not copied, taking up unnecessary space.
+                    call rm -rf "${TF_DEST_DIR}/${lib_full_name}"*
+                    # Copy TensorFlow libraries, preserving symlinks.
+                    call find "${TF_LIB_DIR}" \( -regex ".*${lib_full_name}[0-9.]*" \) -exec cp -a -v {} "${TF_DEST_DIR}" \;
                 done
                 continue
                 ;;