[libcxxabi] Enable LLVM_ENABLE_WARNINGS when building standalone out of tree

[mstorsjo]

Summary

When built within the llvm runtimes directory, the runtimes CMakeLists.txt adds the same.

[EricWF]

Does libc++ need a similar change?

[mstorsjo]

In D56979#1364855, @EricWF wrote:

Does libc++ need a similar change?

No - for reasons I'm not familiar with entirely, libc++ doesn't include HandleLLVMOptions. The last commit where HandleLLVMOptions was included was SVN r243591 ("Attempt to fix build issues introduced by r243574"), while it was gone in SVN r243593 ("Revert recent CMake changes again due to failing compiler-rt builds"). However libunwind needs the same (D56981).

cmake/Modules/HandleOutOfTreeLLVM.cmake

if (WIN32)
       set(LLVM_ON_UNIX 0)
       set(LLVM_ON_WIN32 1)
     else()
       set(LLVM_ON_UNIX 1)
       set(LLVM_ON_WIN32 0)
     endif()
   endif()
   if (LLVM_FOUND)
+    # Enable warnings, otherwise -w gets added to the cflags by HandleLLVMOptions.
+    set(LLVM_ENABLE_WARNINGS ON)
     include(AddLLVM OPTIONAL)
     include(HandleLLVMOptions OPTIONAL)
   endif()
 
   # LLVM Options --------------------------------------------------------------
   if (NOT DEFINED LLVM_INCLUDE_TESTS)
     set(LLVM_INCLUDE_TESTS ${LLVM_FOUND})
   endif()