[clang] Correct source locations for instantiations of function templates.

This change corrects some cases where the source location for an
instantiated specialization of a function template or a member function
of a class template was assigned the location of a non-defining
declaration rather than the location of the definition the
specialization was instantiated from.

Fixes https://github.com/llvm/llvm-project/issues/26057

Reviewed By: cor3ntin

Differential Revision: https://reviews.llvm.org/D64087