[CMake] Support passing arguments to build tool for external projects

[andrewng]

Summary

Add CMake variable LLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS to allow
arguments to be passed to the native tool used in CMake --build
invocations for external projects.

Can be used to pass extra arguments for enhanced versions of build
tools, e.g. distributed build options.

[MaskRay]

(I don't know much about CMake :/)

[andrewng]

Ping.

[beanz]

One minor nit, otherwise looks good.

llvm/CMakeLists.txt

179

We should probably mark this as an advanced option since I don't expect most users will use it.

[andrewng]

Address review comments.

[phosek]

LGTM

[beanz]

LGTM

[andrewng]

llvm/CMakeLists.txt

179

Thanks for the suggestion and I've also now learnt about mark_as_advanced.