rollup merge of #24945: pnkfelix/fixes-for-dash-g-handling

Fixes for -g handling

First:
 * decouples our handling of `-g` for the test suite from our handling of `-g` for the rest of the compiler/stdlib building.
 * Namely, if you do `--enable-debug` or `--enable-debuginfo`, that should only affect `rustc` and the standard library crates; the tests should all continue to compile without `-g` unless:
   * you pass `--enable-debuginfo-tests`, or
   * the test itself requests the `-g` option (e.g. via a `// compile-flags: -g` embedded comment).

Second:
 * Makes `rustc` more flexible in that it now accepts multiple occurrences of `-g -g`
 * (as a drive-by, I gave `-O` the same treatment: multiple occurrences of `-O` are treated as synonymous as a single occurrence of `-O`.

Fix #24937