[modules] When a #include is mapped to a module import and appears somewhere

other than the top level, we issue an error. This breaks a fair amount of C++
code wrapping C libraries, where the C library is #included within a namespace
/ extern "C" combination, because the C library (probably) includes C++
standard library headers which may be within modules.

Without modules, this setup is harmless if (and *only* if) the corresponding
standard library module was already included outside the namespace, so
downgrade the error to a default-error extension in that case, so that it can
be selectively disabled for such misbehaving libraries.

llvm-svn: 253398