Revert "[modules] Fix 37878; Autoload subdirectory modulemaps with specific LangOpts"

This reverts commit f40124d4f05ecf4f880cf4e8f26922d861f705f3 / r336660.

This change shouldn't be affecting `@import` behavior, but turns out it is:
https://ci.swift.org/view/swift-master-next/job/oss-swift-incremental-RA-osx-master-next/2800/consoleFull#-12570166563122a513-f36a-4c87-8ed7-cbc36a1ec144

Working on a reduced testcase for this, reverting in the meantime.

rdar://problem/42102222

llvm-svn: 336920