[ARM64][Windows] Fix local stack size for funclets

The comment was misplaced, and the code didn't do what the comment indicated,
namely ignoring the varargs portion when computing the local stack size of a
funclet in emitEpilogue.  This results in incorrect offset computations within
funclets that are contained in vararg functions.

Differential Revision: https://reviews.llvm.org/D55096

llvm-svn: 348222