COFF: Do not assign linker-weak symbols to selectany comdat sections.

COFF: Do not assign linker-weak symbols to selectany comdat sections.

It is mandatory to specify a comdat in order to receive comdat semantics
for a symbol. We were previously getting this wrong in -function-sections
mode; linker-weak symbols were being emitted in a selectany comdat. This
change causes such symbols to use a noduplicates comdat instead, fixing
the inconsistency.

Also correct an inaccuracy in the docs.

Differential Revision: http://reviews.llvm.org/D10828

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241103 91177308-0d34-0410-b5e6-96231b3b80d8