Merge pull request #19272 from mppf/fix-clang-static

Fix problem with static link order with static clang and llvm

This resolves a problem on CentOS 7 which has an error like this:

```
: CommandLine Error: Option 'enable-vfe' registered more than once!
LLVM ERROR: inconsistency in registered CommandLine options
```

We have some evidence that this error was caused by statically linking
LLVM libraries but dynamically linking clang-cpp. For one thing, on that
system, changing to statically linking both solved this issue.

This PR changes chpl_llvm.py to ask `llvm-config` if it has shared
libraries available (with `llvm-config --shared-mode`) and if not it will
use the static linking approach for both the LLVM and clang dependencies.

Additionally, when using static linking for LLVM and clang, on CentOS 7,
the linker was picky about the order. Since the link line should have the
things depended upon later, this PR puts the `-l` flags for clang
libraries before the `-l` flags for LLVM libraries. On Mac OS X with
Homebrew LLVM, we were using the same (arguably incorrect) `-l` ordering
but for some reason the linker there did not complain.

Note that we statically link both LLVM and clang on Mac OS X (since PR
#18727). This PR leaves it this way. Historically, there have been
problems with upstream LLVM and Homebrew in building a dynamic library
that works with `llvm-config` (in particular the versioned file, like
`libLLVM-11.dylib`, is missing). See [this LLVM bug
report](llvm/llvm-project#39599). I expect that
we will be able to allow dynamic linking on Mac OS X once we are able to
establish that the issue is resolved.

We are statically linking with CHPL_LLVM=bundled and this PR does not
change that.

Reviewed by @Maxrimus - thanks!

- [x] 'hello' builds and runs with system LLVM on Ubuntu 21
- [x] 'hello' builds and runs with system LLVM on SLES 12
- [x] 'hello' builds and runs with system LLVM on Mac OS X with Homebrew
- [x] 'hello' builds and runs with bundled LLVM on Ubuntu 21
- [x] full local testing

util/chplenv/chpl_llvm.py

@@ -630,6 +630,18 @@ def get_host_link_args():
         if host_platform == 'darwin':
             llvm_dynamic = False
 
+        shared_mode = run_command([llvm_config, '--shared-mode'])
+
+        if shared_mode.strip() == 'static':
+            llvm_dynamic = False
+
+        # Make sure to put clang first on the link line
+        # because depends on LLVM libraries
+        if llvm_dynamic:
+            system.append('-lclang-cpp')
+        else:
+            system.extend(clang_static_libs)
+
         libdir = run_command([llvm_config, '--libdir'])
         if libdir:
             libdir = libdir.strip()
@@ -642,10 +654,6 @@ def get_host_link_args():
         if ldflags:
             system.extend(filter_llvm_link_flags(ldflags.split()))
 
-        if llvm_dynamic:
-            system.append('-lclang-cpp')
-        else:
-            system.extend(clang_static_libs)
 
     elif llvm_val == 'bundled':
         # Link statically for now for the bundled configuration