[modules] Make the include guard optimization fire a bit more when considering

re-entering a modular header.

When we do the include guard check, we're in the visibility state for the file
with the #include; the include guard may not be visible there, but we don't
actually need it to be: if we've already parsed the submodule we're considering
entering, it's always safe to skip it.

llvm-svn: 241135