[Sema][Crash] Correctly handle an non-dependent noexcept expr in function template

It seems that all of the other templated cases are handled correctly,
however the function template case was not correctly handled. This
patch recovers from this condition by setting the function to noexcept
after diagnosing. Previously it simply set NoexceptExpr to null,
which caused an Assert when this was evaluated during substitution.

Differential Revision:https://reviews.llvm.org/D38700

llvm-svn: 315638