[NFC] Refactor the Preprocessor function that handles Macro definitions and rename...

[NFC] Refactor the Preprocessor function that handles Macro definitions and rename Arguments to Parameters in Macro Definitions.
  - Extracted the reading of the tokens out into a separate function.
  - Replace 'Argument' with 'Parameter' when referring to the identifiers of the macro definition (as opposed to the supplied arguments - MacroArgs - during the macro invocation).

This is in preparation for submitting patches for review to implement __VA_OPT__ which will otherwise just keep lengthening the HandleDefineDirective function and making it less comprehensible.

Thanks!

llvm-svn: 308157