The fix for regression bug 1482810 moves WARNINGS_CFLAGS into COMPILE_FLAGS and hides warnings generated by C code in Rust crates (by removing warnings flags from CFLAGS when compiling Rust libraries). We should do the same thing for WARNINGS_CXXFLAGS, for the same reason: C++ code in Rust crates is invariably third-party code, and we generally aren't able to fix warnings in it, so it shouldn't clutter the build output for developers. I tried to do that in my fix for bug 1482810, but I ran into some build failures that I couldn't figure out at the time. The fix should be mostly similar to what I did in that bug, however. And the upstream change that I needed to land in the cc crate observes both CFLAGS and CXXFLAGS to determine whether to enable warnings by default. So it should be possible to hide C++ warnings without further upstream changes.

MozReview-Commit-ID: Lu0D4or8fCx

Here's a tryserver run with the WIP patch in https://bugzilla.mozilla.org/attachment.cgi?id=9005508: https://treeherder.mozilla.org/#/jobs?repo=try&revision=37cfc42c1c4bf1184e0bd1531d4bdc2e59b892e6

Triaging, assigning to Myk since he attached patches.