And move LLVM_DIR and LLVM_CONFIG_DIR into the llvm's CMakeLists.txt

Bug: b/145758253
Bug: b/152339534
Change-Id: I29e555e09b82dcf77cd9e9d57b5fb8a477473bbe
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43008
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>