libclang can be built in shared or static mode. On Windows, with LLVM_ENABLE_PIC=OFF, it was built in neither mode, leading to clients of libclang (c-index-test, c-arcmt-test) failing to link with it set.
Since PIC isn't really a thing on Windows, build libclang in shared mode when LLVM_ENABLE_PIC=OFF on Windows. This is also somewhat symmetric with the existing ENABLE_STATIC a few lines down.