Reword -Winternal-linkage-in-inline, and rename it to -Wstatic-in-inline.

Now that this is a C-only warning, we can use "static" instead of "internal
linkage", which is a term developers are probably more familiar with.
This makes for a better warning message. The warning name was changed to match,
since "internal linkage" is not mentioned in the warning text anymore.

llvm-svn: 158853