MC Win64: Put unwind info for COMDAT code into the same COMDAT group

Summary:
This fixes a long standing issue where we would emit many little .text
sections and only one .pdata and .xdata section. Now we generate one
.pdata / .xdata pair per .text section and associate them correctly.

Fixes PR19667.

Reviewers: majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5181

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217176 91177308-0d34-0410-b5e6-96231b3b80d8