Update Append Argument to more efficiently traverse tokens

This function was previously making (correct) assumptions
without complete knowledge of MacroArgs guarantees for
Arguments.  After going through Macro Args a bunch, I'd
corrected the getNumArguments (and changed its name), 
however didn't realize this was depending on the behavior.

This patch has version that depends on the corrected 
getNumMacroArguments's behavior, with the rest checked against
my knowledge of the MacroArgs' token list.  Commiting no-wait
since the test is broken.

llvm-svn: 305434