[MSVC] Mimic MSVC whitespace collapse for incompatible token pasting

In public MS headers for XAudio, clang would fail to generate a valid UUID due to the UUID components being combined with the '-' UUID separators. Clang would attempting to recover but would preserve the leading whitespace from the tokens after each failed paste leading to spaces creeping into the UUID and causing an error in the __declspace(uuid()) parsing.

Reference: Microsoft DirectX SDK (June 2010)\Include\XAudio2.h(51)

Resolves http://llvm.org/pr23071

llvm-svn: 235186