[Preprocessor] When checking if we can concatenate two tokens, check
if they were already concatenated in source using the spelling locations
even if they came from a macro expansion.

This fixes an issue where a GUID passed as macro argument ends up
malformed after preprocessing because we added spaces inside it.

rdar://13016645

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173826 91177308-0d34-0410-b5e6-96231b3b80d8