A follow-on to b436825948 where
-m${platform}-version-min was traded for -target. The lit substitution
for %target-clang was relying on -arch and -version-min to specify the
intended platform version to the linker. With the removal of
version-min, only arch was given, so a fallback platform was selected.
For the iPhone simulator, for example, this is iOS 10.99 despite there
being no such simulator for that deployment version. Provide the variant
triple everywhere instead. This has the added benefit of unifying the
special paths catalyst takes.
Fixes rdar://73174820
This is the only dependency it has on libswiftCore. Looking this up at runtime allows its use in programs that don't link libswiftCore but might eventually load and run Swift code, such as xctest.
While we're in there, enable tests in files ending with `.c`.
rdar://problem/55274114
