Remove -Winternal-linkage-in-inline in C++.

It's very easy for anonymous external linkage to propagate in C++ through
return types and parameter types. Likewise, it's possible that a template
containing an inline function is only used with parameters that have internal
linkage. Actually diagnosing where the internal linkage comes from is fairly
difficult (both to locate and then to print nicely). Finally, since we only
have one translation unit available, we can't even prove that any of this
violates the ODR.

This warning needs better-defined behavior in C++ before it can really go in.

Rewording of the C warning (which /is/ specified by C99) coming shortly.

llvm-svn: 158836