CMake: Provide LLVM_PLUGIN_EXT definition

This is needed to support the addition of tests for clang loadable plugins.

In clang, plugins are built as modules (bundles on OS X) rather than dynamic
libraries (dylib) so the build system needs to inform lit of the actual
file extension in use, typically '.so' on Unix and '.dll' on Windows.

(LLVM itself should probably switch to this scheme to fix PR14903 once and for
all.)

No change in build output or functionality intended.