Simplify linkage code for static local vars.

The key insight here is that weak linkage for a static local variable
should always mean linkonce_odr, because every file that needs it will
generate a definition.  We don't actually care about the precise linkage
of the parent context.  I feel a bit silly that I didn't realize this before.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185381 91177308-0d34-0410-b5e6-96231b3b80d8