Several tests related to indexing system modules were taking a considerable
amount of time (100+ seconds in the worst case) indexing the standard library.
This adds a frontend option to skip it and updates those tests to pass it.
Rely on the `ls` behaviour of returning a non-zero if there is no
matching pattern. This should help make the test pass on Windows
and the other targets.
Previously 'isSystemModule()' returns true only if the module is:
- Standard library
- Clang module and that is `IsSystem`
- Swift overlay for clang `IsSystem` module
Now:
- Clang module and that is `IsSystem`; or
- Swift overlay for clang `IsSystem` module
- Swift module found in either of these directories:
- Runtime library directoris (including stdlib)
- Frameworks in `-Fsystem` directories
- Frameworks in `$SDKROOT/System/Library/Frameworks/` (Darwin)
- Frameworks in `$SDKROOT/Library/Frameworks/` (Darwin)
rdar://problem/50516314
