Instantiate exception specifications when instantiating function types (other

Instantiate exception specifications when instantiating function types (other
than the type of a function declaration). We previously didn't instantiate
these at all! This also covers the pathological case where the only mention of
a parameter pack is within the exception specification; this gives us a second
way (other than alias templates) to reach the horrible state where a type
contains an unexpanded pack, but its canonical type does not.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217995 91177308-0d34-0410-b5e6-96231b3b80d8