Modular Codegen: Don't home/modularize static functions in headers

Consistent with various workarounds in the backwards compatible modules
that allow static functions in headers to exist, be deduplicated to some
degree, and not generally fail right out of the gate... do the same with
modular codegen as there are enough cases (including in libstdc++ and in
LLVM itself - though I cleaned up the easy ones) that it's worth
supporting as a migration/backcompat step.

Simply create a separate, internal linkage function in each object that
needs it. If an available_externally/modularized function references a
static function, but the modularized function is eventually dropped and
not inlined, the static function will be dropped as unreferenced.

llvm-svn: 317274