Another default arg cleanup

[Nathan Sidwell]

Richard,
in working on 18432 I've another cleanup patch.

1) code in Parser::ParseLexedMethodDeclaration uses 'LM.DefaultArgs[I].Param' a 
lot (and my fix for 18432 will add more).  It seemed better to break that out 
into a local var.  It's probably very hard for the compiler to do that 
localization itself, as it'd have to prove that LM.DefaultArgs[I].Param was not 
mutated by any function call -- I'd put it in the 'impossible for practical 
purposes' category.

2) Code in Sema::MergeCXXFunctionDecl looks to find the defining default arg in 
order to issue a duplicate default diagnostic.  However the current loop probes 
backwards until it finds a param without a default -- and then uses the one just 
previously found.  However, it seems simpler to just check 
hasInheritedDefaultArg and stop on the first param without that.

tested on x86-linux, ok?

nathan
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