Change -Winternal-linkage-in-inline from ExtWarn to Warning in C++.

Per post-commit review, it's not appropriate to use ExtWarn in C++, because
we can't prove that the inline function will actually be defined in more than
one place (and thus we can't prove that this violates the ODR).

This removes the warning entirely from uses in the main source file in C++.

llvm-svn: 158689