Auto merge of #151683 - Zalathar:compiler-kind, r=jieyouxu

compiletest: Add `CompilerKind` to distinguish between rustc and rustdoc

This PR slightly improves `TestCx::make_compile_args` by using `CompilerKind` to explicitly distinguish between rustc and rustdoc.

The resulting code could still use more attention, but I think this is a good incremental improvement.

There *should* (hopefully) be no change to overall compiletest behaviour.

Author: bors