When a tag is declared in prototype scope in C, if we've decided that it

redeclares an existing tag but are creating a new declaration anyway (because
it has attributes or changes the visibility of the name), don't warn that it
won't be visible outside the current scope. That's not true.

Also narrow down the set of cases where we create these extra declarations when
building modules; previously, all tag declarations but the first in a module
header would get this treatment if -fmodules-local-submodule-visibility. (This
isn't a functional change, but we try to avoid creating these extra
declarations whenever we can.)

llvm-svn: 257403