Better source location for -Wignored-qualifiers on trailing return types

Better source location for -Wignored-qualifiers on trailing return types

We collect the source location of a trailing return type in the parser,
improving the location for regular functions and providing a location
for lambdas, where previously there was none.

Fixes PR47732.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D90129