Use comdats to avoid double initialization of weak data

Use comdats to avoid double initialization of weak data

Initializers of global data that can appear multiple TUs (static data
members of class templates or __declspec(selectany) data) are now in a
comdat group keyed on the global variable being initialized.  On
non-Windows platforms, this is a code size and startup time
optimization.  On Windows, this is necessary for ABI compatibility with
MSVC.

Fixes PR16959.

Reviewers: rsmith

Differential Revision: http://reviews.llvm.org/D3811

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209555 91177308-0d34-0410-b5e6-96231b3b80d8