PR27549: fix bug that resulted in us giving a translation-unit-scope variable a

mangled name if it happened to be declared in an 'extern "C++"' context. This
also causes us to use the '_ZL' mangling rather than the '_Z' mangling for
internal-linkage entities that are wrapped in a language linkage construct.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267969 91177308-0d34-0410-b5e6-96231b3b80d8