Auto merge of #126197 - jieyouxu:rmake-must-use, r=Kobzol

run-make: annotate library with `#[must_use]` and enforce `unused_must_use` in rmake.rs

This PR adds `#[must_use]` annotations to functions of the `run_make_support` library where it makes sense, and adjusts compiletest to compile rmake.rs with `-Dunused_must_use`.

The rationale is that it's highly likely that unused `#[must_use]` values in rmake.rs test files are bugs. For example, unused fs/io results are often load-bearing to the correctness of the test and often unchecked fs/io results allow the test to silently pass where it would've failed if the result was checked.

This PR is best reviewed commit-by-commit.

try-job: test-various
try-job: x86_64-msvc

Author: bors