Use FinishThunk to finish musttail thunks

FinishThunk, and the invariant of setting and then unsetting
CurCodeDecl, was added in 7f416cc4 (2015). The invariant didn't
exist when I added this musttail codepath in ab2090d1 (2014).
Recently in 28328c37, I started using this codepath on non-Windows
platforms, and users reported problems during release testing (PR44987).

The issue was already present for users of EH on i686-windows-msvc, so I
added a test for that case as well.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D76444