Win64: Don't emit unwind info for "leaf" functions (PR30337)

According to MSDN (see the PR), functions which don't touch any callee-saved
registers (including %rsp) don't need any unwind info.

This patch makes LLVM not emit unwind info for such functions, to save
binary size.

Differential Revision: https://reviews.llvm.org/D24748

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282185 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed