[CMake] Enable warnings for runtimes

[cryptoad]

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.

[phosek]

The only other way I could think is to add another option, e.g. LLVM_RUNTIMES_ENABLE_WARNING which would be handled by runtimes/CMakeLists.txt and turned into LLVM_ENABLE_WARNINGS if set, but if this works I think it's perfectly sufficient.

[cryptoad]

Currently not compatible with D45997's COMPILER_RT_ENABLE_WERROR ON.

llvm/trunk/runtimes/CMakeLists.txt

if(LLVM_HAS_NODEFAULTLIBS_FLAG)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
     if(LLVM_HAS_C_LIB)
       list(APPEND CMAKE_REQUIRED_LIBRARIES c)
     endif()
   endif()
 
   # 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)
   include(HandleLLVMOptions)
 
   set(CMAKE_REQUIRED_FLAGS ${SAFE_CMAKE_REQUIRED_FLAGS})
   set(CMAKE_REQUIRED_LIBRARIES ${SAFE_CMAKE_REQUIRED_LIBRARIES})