[preprocessor] Allow comparing two macro definitions syntactically instead of only lexically.

Syntactically means the function macro parameter names do not need to use the same
identifiers in order for the definitions to be considered identical.

Syntactic equivalence is a microsoft extension for macro redefinitions and we'll also
use this kind of comparison to check for ambiguous macros coming from modules.

rdar://13562254

llvm-svn: 178671