Fix -Wunused-comparison for comparisons in arguments to function-like macros.

Previously, -Wunused-comparison ignored comparisons in both macro bodies and
macro arguments, but we would still emit a -Wunused-value warning for either.
Now we correctly emit -Wunused-comparison for expressions in macro arguments.

Also, add isMacroBodyExpansion to SourceManager, to go along with
isMacroArgExpansion.