The base change moved the LLVM rules out to a separate CMakeLists.txt file, while trying to keep the behaviour as close as possible to original. `LLVM_INCLUDE_DIR` is not needed to be exposed if we made it a public include directory on the llvm target, so do that.

Bug: b/145758253
Bug: b/152339534
Change-Id: I6e409da409d9a28847ac5bb4074899d70bb38145
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42971Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>