cmd/compile: let compiler downgrade its own concurrency
This gets the Go command out of the business of thinking it understands
compiler debug flags, and allows the compiler to turn down its worker
concurrency instead of failing and forcing the user to do the very
same thing. Debug flags that are obviously safe for concurrency
(at least to me) are tagged; probably there's more.
Change-Id: I59bb19861d8a654a9cfd2364ee78c8628212f82e
Reviewed-on: https://go-review.googlesource.com/c/go/+/448359 Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>