All .xdata sections are eligble for ICF

Summary:
Many small functions have identical unwind info because they push the
same sets of CSRs in the same order and have the same stack and prologue
size. The VC linker merges duplicate .xdata, and so should LLD.

This reduces the .xdata section size of clang.exe from 1.8MB to 94KB.

Reviewers: pcc, ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40160

llvm-svn: 318547