[clang] Build clang-shlib on mingw

[vchuravy]

Summary

Currently we exclude the libclang-cpp cmake files
on any non-unix platform, this excludes mingw32.

I suspect that the goal was to exclude MSVC only.
For the LLVM shlib we throw an error if someone tries
to build the shared library, whereas I stuck to the pattern
here that a misconfiguration causes an early return.

[vchuravy]

Some additional context. We [Julia] is cross-compiling LLVM from Linux to mingw32
as part of an automatic buildsystem for our binary dependencies. The corresponding PR is https://github.com/JuliaPackaging/Yggdrasil/pull/417

[merge_guards_bot]

Unit tests: pass. 62088 tests passed, 0 failed and 784 were skipped.

clang-tidy: unknown.

clang-format: pass.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[mstorsjo]

This actually is done (almost) like this in current git master, see D87547 and D89225.

clang/tools/CMakeLists.txt

 add_clang_subdirectory(clang-offload-bundler)
 add_clang_subdirectory(clang-offload-wrapper)
 add_clang_subdirectory(clang-scan-deps)
 
 add_clang_subdirectory(c-index-test)
 
 add_clang_subdirectory(clang-rename)
 add_clang_subdirectory(clang-refactor)
-if(UNIX)
-  add_clang_subdirectory(clang-shlib)
-endif()
 
 if(CLANG_ENABLE_ARCMT)
   add_clang_subdirectory(arcmt-test)
   add_clang_subdirectory(c-arcmt-test)
 endif()
 
 if(CLANG_ENABLE_STATIC_ANALYZER)
   add_clang_subdirectory(clang-check)

clang/tools/clang-shlib/CMakeLists.txt

 # Building libclang-cpp.so fails if LLVM_ENABLE_PIC=Off
 if (NOT LLVM_ENABLE_PIC)
   return()
 endif()
 
+# Building libclang-cpp.so may not work on MSVC
+if (MSVC)
+  return()
+endif()
+
 get_property(clang_libs GLOBAL PROPERTY CLANG_STATIC_LIBS)
 
 foreach (lib ${clang_libs})
   if(XCODE)
     # Xcode doesn't support object libraries, so we have to trick it into
     # linking the static libraries instead.
     list(APPEND _DEPS "-force_load" ${lib})
   else()