[codeview] Use comdats for debug info describing comdat functions

Summary:
This allows the linker to discard unused symbol information for comdat
functions that were discarded during the link. Before this change,
searching for the name of an inline function in the debugger would
return multiple results, one per symbol subsection in the object file.
After this change, there is only one result, the result for the function
chosen by the linker.

Reviewers: zturner, majnemer

Subscribers: aaboud, amccarth, llvm-commits

Differential Revision: http://reviews.llvm.org/D20642

llvm-svn: 270792