NFC: Rename MCSafeSEHFragment to MCSymbolIdFragment

Summary:
This fragment emits a symbol ID and will be useful for more than just Safe SEH
tables (e.g., I plan to re-use it for Control Flow Guard tables).  This is
simply a rename refactor.

Reviewers: rnk

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D39770

llvm-svn: 317703