[CMake] Enable warnings for runtimes

Summary:
`HandleLLVMOptions` adds `-w` to the cflags if `LLVM_ENABLE_WARNINGS` is not on.
With `-w`, `check_cxx_compiler_flag` doesn't error out for unsupported flags
(for example `-mcrc` on x86_64), and those flags end up being detected as
working - and really they aren't.

I am not entirely sure what the best way to solve this is, but setting
`LLVM_ENABLE_WARNINGS` prior to including `HandleLLVMOptions` does the job.

Reviewers: phosek, beanz

Reviewed By: phosek

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D46079

llvm-svn: 331042

Author: Kostya Kortchinsky

llvm/runtimes/CMakeLists.txt

@@ -109,6 +109,9 @@ if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
 
   # Avoid checking whether the compiler is working.
   set(LLVM_COMPILER_CHECKED ON)
+  # Enable warnings, otherwise -w gets added to the cflags by HandleLLVMOptions
+  # resulting in unjustified successes by check_cxx_compiler_flag.
+  set(LLVM_ENABLE_WARNINGS ON)
 
   # Handle common options used by all runtimes.
   include(AddLLVM)