Xcode and other build systems currently link the compatibility libraries into every executable
or dynamic library, which can cause a chain of objc_getClass compatibility hacks to pile up if
a program loads a lot of dynamic libraries. In the static constructor that installs the
objc_getClass compatibility shim, check whether it's running on behalf of the main executable
before installing the shim.
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
Add code to the 5.0 compatibility library that scans for conformances pointing to a NULL type and rewrites them to point to a dummy type that the 5.0 protocol conformance checking code will safely ignore.
