Add proper use of LLVM_RELEASE_VERSION for locating libraries.

Bug: http://b/31532493

As I have been updating to LLVM ~4.0, there are numerous locations where
3.8 is hard-coded, and unable to be overridden. This change adds a new
function for getting the ClangShortVersion(), which can look at
environment variables that override this value (particularly useful for
two-stage compilation, where version numbers change between builds).

Change-Id: Iaec19908cf47b3b2317844d6e1329f7bcd81f022