Merging r278471:

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r278471 | gbiv | 2016-08-11 21:12:31 -0700 (Thu, 11 Aug 2016) | 11 lines

[Sema] Fix a crash on variadic enable_if functions.

Currently, when trying to evaluate an enable_if condition, we try to
evaluate all arguments a user passes to a function. Given that we can't
use variadic arguments from said condition anyway, not converting them
is a reasonable thing to do. So, this patch makes us ignore any varargs
when attempting to check an enable_if condition.

We'd crash because, in order to convert an argument, we need its
ParmVarDecl. Variadic arguments don't have ParmVarDecls.

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llvm-svn: 278479