Allow internal decls in inline functions if the function is in the main file.

This handles the very common case of people writing inline functions in their
main source files and not tagging them as inline. These cases should still
behave as the user intended. (The diagnostic is still emitted as an extension.)

I'm reworking this code anyway to account for C++'s equivalent restriction in
[basic.def.odr]p6, but this should get some bots back to green.

llvm-svn: 158666