Add support for LLVM_REPOSITORY_STRING.

Without this, vendor Clang builds can occasionally pick up an arbitrary
mirror location for the repository (at least on Android builds). This
allows vendor builds to be more reproducible.

Bug: http://b/68397894
Test: toolchain/llvm_android/build.py and run clang --version.
Change-Id: Ib9f3932421973aa8c979de08c2bf8aca22e79b38
2 files changed