public LLVM CLANG headers exposed MSVC warnings, better use a testcase to catch all warnings of public headers.

[lygstate]

C:/mesa-deps/include\llvm/Option/ArgList.h(135): error C2220: the following warning is treated as an error
C:/mesa-deps/include\llvm/Option/ArgList.h(135): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\clang/Serialization/SourceLocationEncoding.h(111): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\clang/Serialization/SourceLocationEncoding.h(177): warning C4146: unary minus operator applied to unsigned type, result still unsigned

C:/mesa-deps/include\llvm/TargetParser/SubtargetFeature.h(58): error C2220: the following warning is treated as an error
C:/mesa-deps/include\llvm/TargetParser/SubtargetFeature.h(58): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/Analysis/LoopAccessAnalysis.h(309): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/CodeGen/SelectionDAGNodes.h(242): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/CodeGen/SelectionDAGNodes.h(248): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/CodeGen/SelectionDAGNodes.h(1179): warning C4146: unary minus operator applied to unsigned type, result still unsigned

C:/mesa-deps/include\llvm/DebugInfo/DIContext.h(195): error C2220: the following warning is treated as an error
C:/mesa-deps/include\llvm/DebugInfo/DIContext.h(195): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/DebugInfo/DIContext.h(196): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/DebugInfo/DIContext.h(224): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/DebugInfo/DIContext.h(226): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/CodeGen/SelectionDAGNodes.h(242): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/CodeGen/SelectionDAGNodes.h(248): warning C4146: unary minus operator applied to unsigned type, result still unsigned
C:/mesa-deps/include\llvm/CodeGen/SelectionDAGNodes.h(1179): warning C4146: unary minus operator applied to unsigned type, result still unsigned

[RKSimon]

Ideally we'd drop -wd4146 entirely from HandleLLVMOptions.cmake and warn for it internally as well as the headers.