Rollup merge of #78524 - tmiasko:source-files-borrow, r=Aaron1011

Avoid BorrowMutError with RUSTC_LOG=debug

```console
$ touch empty.rs
$ env RUSTC_LOG=debug rustc +stage1 --crate-type=lib empty.rs
```

Fails with a `BorrowMutError` because source map files are already
borrowed while `features_query` attempts to format a log message
containing a span.

Release the borrow before the query to avoid the issue.

src/test/ui/auxiliary/rustc-rust-log-aux.rs

@@ -0,0 +1 @@
+// rustc-env:RUSTC_LOG=debug

src/test/ui/rustc-rust-log.rs

@@ -8,7 +8,7 @@
 // dont-check-compiler-stdout
 // dont-check-compiler-stderr
 // compile-flags: --error-format human
-
+// aux-build: rustc-rust-log-aux.rs
 // rustc-env:RUSTC_LOG=debug
 
 fn main() {}