The motivation for this is to enable correct detection of dlopen() on Android.
Android does not provide a static version of libdl, so if we add the -static flag
after performing the check, it will succeed even though subsequent link steps
will fail. With this change we correctly detect the absence of libdl in a
LLVM_BUILD_STATIC build on Android.
The link itself still does not succeed because the code does not check the result
of this check properly, but I plan to fix that in a separate change.