[clang] Set function attributes on SEH filter functions correctly.

Summary:
When compiling with -munwind-tables, the SEH filter funclet needs the uwtable
function attribute, which gets automatically added if we use
SetInternalFunctionAttributes.  The filter funclet is internal so this seems
appropriate.

Reviewers: rnk

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72786