[clang] Fix __try/__finally blocks in C++ constructors.

We were crashing trying to convert a GlobalDecl from a
CXXConstructorDecl.  Instead of trying to do that conversion, just pass
down the original GlobalDecl.

I think we could actually compute the correct constructor/destructor
kind from the context, given the way Microsoft mangling works, but it's
simpler to just pass through the correct constructor/destructor kind.

Differential Revision: https://reviews.llvm.org/D136776