Before this change, we would emit the warning only for swiftmodules; however,
it may be the case that only a swiftinterface (of a different arch) is present
but no swiftmodule is present.
Fixes rdar://problem/50905075.
The standard library (and other Swift modules built by our CMake build system)
has been building module files with an architecture only (e.g., x86_64.swiftmodule)
rather than a proper module triple (x86_86-apple-macosx10.15,
x86_64-apple-ios13.0-simulator, etc.), unlike every other build
system. There are hacks in the compiler and other tools to cope with
this unnecessary build difference. Fix the module file names so we'll
be able to remove the hacks later.
Fixes rdar://problem/49071536.