Sema: Simplify the check if a method returns an instance of the class.

Just checking if the parent of the method is the same as the return type
should be sufficient. Also fixes PR17587.
Assert on duplicate registration. Don't depend on function pointer equality.

Before this patch we would assert when building llvm as multiple shared
libraries (cmake's BUILD_SHARED_LIBS). The problem was the line

if (T.AsmStreamerCtorFn == Target::createDefaultAsmStreamer)

which returns false because of -fvisibility-inlines-hidden. It is easy
to fix just this one case, but I decided to try to also make the
registration more strict. It looks like the old logic for ignoring
followup registration was just a temporary hack that outlived its
usefulness.

This patch converts the ifs to asserts, fixes the few cases that were
registering twice and makes sure all the asserts compare with null.

Thanks for Joerg for reporting the problem and reviewing the patch.
Update comment.