Support -Winternal-linkage-in-inline in C++ code.

This includes treating anonymous namespaces like internal linkage, and allowing
const variables to be used even if internal. The whole thing's been broken out
into a separate function to avoid nested ifs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158683 91177308-0d34-0410-b5e6-96231b3b80d8