Don't make assumptions about the mangling of static functions in extern "C"

Don't make assumptions about the mangling of static functions in extern "C"
blocks. We still don't have consensus if we should try to change clang or
the standard, but llvm should work with compilers that implement the current
standard and mangle those functions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175267 91177308-0d34-0410-b5e6-96231b3b80d8