[ARM64][Windows] Share unwind codes between epilogues

There are cases where we have multiple epilogues that have the exact same unwind
code sequence.  In that case, the epilogues can share the same unwind codes in
the .xdata section.  This should get us past the assert "SEH unwind data
splitting not yet implemented" in many cases.

We still need to add support for generating multiple .pdata/.xdata sections for
those functions that need to be split into fragments.

Differential Revision: https://reviews.llvm.org/D56813

llvm-svn: 351421