Arch Linux

Description:
I was unable to build Zig (https://ziglang.org) unless I built llvm and clang myself.

The reason is that the only supported clang configuration is statically linking
with libclang*.a files. Using BUILD_SHARED_LIBS=ON is not a supported configuration
for any of the llvm tools as stated in [building LLVM with CMake](https://llvm.org/docs/CMake.html):
```
BUILD_SHARED_LIBS is only recommended for use by LLVM developers. If you want to build LLVM as a shared library, you should use the LLVM_BUILD_LLVM_DYLIB option.
```

So Zig needs the libclang*.a files to build, but those aren't available in the clang package:
```
$ ls /usr/lib/libclang*.a
ls: cannot access '/usr/lib/libclang*.a': No such file or directory
```
Only the libclang*.so files are available
```
$ ls /usr/lib/libclang*.so
/usr/lib/libclangAnalysis.so /usr/lib/libclangFrontendTool.so /usr/lib/libclangStaticAnalyzerCheckers.so /usr/lib/libclangTidyObjCModule.so
/usr/lib/libclangApplyReplacements.so /usr/lib/libclangHandleCXX.so /usr/lib/libclangStaticAnalyzerCore.so /usr/lib/libclangTidyPerformanceModule.so
/usr/lib/libclangARCMigrate.so /usr/lib/libclangHandleLLVM.so /usr/lib/libclangStaticAnalyzerFrontend.so /usr/lib/libclangTidyPlugin.so
/usr/lib/libclangASTMatchers.so /usr/lib/libclangIncludeFixerPlugin.so /usr/lib/libclangTidyAbseilModule.so /usr/lib/libclangTidyPortabilityModule.so
/usr/lib/libclangAST.so /usr/lib/libclangIncludeFixer.so /usr/lib/libclangTidyAndroidModule.so /usr/lib/libclangTidyReadabilityModule.so
/usr/lib/libclangBasic.so /usr/lib/libclangIndex.so /usr/lib/libclangTidyBoostModule.so /usr/lib/libclangTidy.so
/usr/lib/libclangChangeNamespace.so /usr/lib/libclangLex.so /usr/lib/libclangTidyBugproneModule.so /usr/lib/libclangTidyUtils.so
/usr/lib/libclangCodeGen.so /usr/lib/libclangMove.so /usr/lib/libclangTidyCERTModule.so /usr/lib/libclangTidyZirconModule.so
/usr/lib/libclangCrossTU.so /usr/lib/libclangParse.so /usr/lib/libclangTidyCppCoreGuidelinesModule.so /usr/lib/libclangToolingASTDiff.so
/usr/lib/libclangDaemon.so /usr/lib/libclangQuery.so /usr/lib/libclangTidyFuchsiaModule.so /usr/lib/libclangToolingCore.so
/usr/lib/libclangDoc.so /usr/lib/libclangReorderFields.so /usr/lib/libclangTidyGoogleModule.so /usr/lib/libclangToolingInclusions.so
/usr/lib/libclangDriver.so /usr/lib/libclangRewriteFrontend.so /usr/lib/libclangTidyHICPPModule.so /usr/lib/libclangToolingRefactor.so
/usr/lib/libclangDynamicASTMatchers.so /usr/lib/libclangRewrite.so /usr/lib/libclangTidyLLVMModule.so /usr/lib/libclangTooling.so
/usr/lib/libclangEdit.so /usr/lib/libclangSema.so /usr/lib/libclangTidyMiscModule.so
/usr/lib/libclangFormat.so /usr/lib/libclangSerialization.so /usr/lib/libclangTidyModernizeModule.so
/usr/lib/libclangFrontend.so /usr/lib/libclang.so /usr/lib/libclangTidyMPIModule.so
```


Additional info:
* package version(s):
A quick look at the history of PKGBUILD, this has been the case at least since clang was split from
llvm in March 2018 with change ca81062dc3ce320dfd7ff52bd40d56ebc304d698.

* link to upstream:
See [D61804](https://reviews.llvm.org/D61804) where I'm investigating a solution
which both static and shared libs can be built.

Steps to reproduce:
Install clang with pacman and try to build a program that needs libclang*.a, such as Zig.