[clang][deps] Substitute clang-scan-deps executable in lit tests

The lit tests for `clang-scan-deps` invoke the tool without going through the substitution system. While the test runner correctly picks up the `clang-scan-deps` binary from the build directory, it doesn't print its absolute path. When copying the invocations when reproducing test failures, this can result in `command not found: clang-scan-deps` errors or worse yet: pick up the system `clang-scan-deps`. This patch adds new local `%clang-scan-deps` substitution.

Reviewed By: lxfind, dblaikie

Differential Revision: https://reviews.llvm.org/D107155