Reapply "[libc] build with -Werror (llvm#73966)" (llvm#74506)

This reverts commit 6886a52.

Most of the errors observed in postsubmit have been addressed. We can
fix-forward the remaining ones.

Link: https://lab.llvm.org/buildbot/#/changes/117129

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -43,6 +43,10 @@ function(_get_common_compile_options output_var flags)
     list(APPEND compile_options "-fno-rtti")
     list(APPEND compile_options "-Wall")
     list(APPEND compile_options "-Wextra")
+    # -DLIBC_WNO_ERROR=ON if you can't build cleanly with -Werror.
+    if(NOT LIBC_WNO_ERROR)
+      list(APPEND compile_options "-Werror")
+    endif()
     list(APPEND compile_options "-Wconversion")
     list(APPEND compile_options "-Wno-sign-conversion")
     list(APPEND compile_options "-Wimplicit-fallthrough")

libc/docs/dev/code_style.rst

@@ -178,3 +178,11 @@ these functions do not call the constructors and destructors of the
 allocated/deallocated objects. So, use these functions carefully and only
 when it is absolutely clear that constructor and destructor invocation is
 not required.
+
+Warnings in sources
+===================
+
+We expect contributions to be free of warnings from the `minimum supported
+compiler versions`__ (and newer).
+
+.. __: https://libc.llvm.org/compiler_support.html#minimum-supported-versions