[ms-cxxabi] Fix assertion on unhandled function template arg types

This code was changed in r158376 to get template argument source info
for better diagnostics, but the current code asserts for any kind of
unsupported template argument before it can issue a diagnostic.  This change
goes back to the Itanium implementation of isTemplate() and puts the argument
index into the diagnostic instead of a source location.

Review URL: http://llvm-reviews.chandlerc.com/D553