Upstream llvm now lives in a mono-repo along with clang, libc, lldb and a whole bunch of other projects (~90k files at time of writing).
SwiftShader only requires the llvm project from this repo, and does not wish to pull in everything else.

This script performs the following:
 * The llvm10-clean branch is fetched and checked out.
 * A sparse checkout of the llvm project is made to a temporary directory.
 * The third_party/llvm-10.0/llvm is replaced with the latest LLVM version.
 * This is committed and pushed.
 * The original branch is checked out again, and a merge from llvm10-clean to
   the original branch is made.

Bug: b/152339534
Change-Id: I75a7bf9e3f4deb7cbf6ff3f7f9428f07b8422800
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42968Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>