Disable compiler launcher on external projects and multi stage clang

When using compiler caching program like ccache, there is no point to
use them on external projects or multi-stage clang builds. As these
builds uses fresh from source code toolchain and will pollute the build
cache. If a compiler launcher is still required, a user can explicity
define `CMAKE_C_COMPILER_LAUNCHER` and `CMAKE_CXX_COMPILER_LAUNCHER` in
`CLANG_BOOTSTRAP_PASSTHROUGH` and `LLVM_EXTERNAL_PROJECT_PASSTHROUGH`
flags to enable compiler launcher in these builds.

Differential Revision: https://reviews.llvm.org/D144603