Apply `simulate-remapped-rust-src-base` even `remap-debuginfo` is set in config.toml

This is really a mess. Here is the situation before this change:

- UI tests depend on not having `rust-src` available. In particular, <https://github.com/rust-lang/rust/blob/3f374128ee3924514aacadf96479e17fee8f9903/tests/ui/tuple/wrong_argument_ice.stderr#L7-L8> is depending on the `note` being a single line and not showing the source code.
- When `download-rustc` is disabled, we pass `-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX` `-Ztranslate-remapped-path-to-local-path=no`, which changes the diagnostic to something like `  --> /rustc/FAKE_PREFIX/library/alloc/src/collections/vec_deque/mod.rs:1657:12`
- When `download-rustc` is enabled, we still pass those flags, but they no longer have an effect. Instead rustc emits diagnostic paths like this: `  --> /rustc/39c6804b92aa202369e402525cee329556bc1db0/library/alloc/src/collections/vec_deque/mod.rs:1657:12`. Notice how there's a real commit and not `FAKE_PREFIX`. This happens because we set `CFG_VIRTUAL_RUST_SOURCE_BASE_DIR` during bootstrapping for CI artifacts, and rustc previously didn't allow for `simulate-remapped` to affect paths that had already been remapped.
- Pietro noticed this and decided the right thing was to normalize `/rustc/<commit>` to `$SRC_DIR` in compiletest: https://github.com/rust-lang/rust/pull/103969/commits/470423c3d2cde3a62d5d4ac23840d734e8145366
- After my change to `x test core`, which rebuilds stage 2 std from source so `build/stage2-std` and `build/stage2` use the same `.rlib` metadata, the compiler suddenly notices it has sources for `std` available and prints those in the diagnostic, causing the test to fail.

This changes `simulate-remapped-rust-src-base` to support remapping paths that have already been remapped, unblocking download-rustc.

Unfortunately, although this fixes the specific problem for
download-rustc, it doesn't seem to affect all the compiler's
diagnostics. In particular, various `mir-opt` tests are failing to
respect `simulate-remapped-path-prefix` (I looked into fixing this but
it seems non-trivial). As a result, we can't remove the normalization in
compiletest that maps `/rustc/<commit>` to `$SRC_DIR`, so this change is
currently untested anywhere except locally.

2 files changed, 5 insertions, 2 deletions

diff --git a/tests/ui/track-diagnostics/track6.rs b/tests/ui/track-diagnostics/track6.rs
index 307e3101849..fc6f5f23d92 100644
--- a/tests/ui/track-diagnostics/track6.rs
+++ b/tests/ui/track-diagnostics/track6.rs
@@ -1,6 +1,9 @@
 // compile-flags: -Z track-diagnostics
 // error-pattern: created at
 
+// Normalize the emitted location so this doesn't need
+// updating everytime someone adds or removes a line.
+// normalize-stderr-test ".rs:\d+:\d+" -> ".rs:LL:CC"
 
 
 pub trait Foo {
diff --git a/tests/ui/track-diagnostics/track6.stderr b/tests/ui/track-diagnostics/track6.stderr
index 1c7537633ff..89438aea9ad 100644
--- a/tests/ui/track-diagnostics/track6.stderr
+++ b/tests/ui/track-diagnostics/track6.stderr
@@ -1,9 +1,9 @@
 error[E0658]: specialization is unstable
-  --> $DIR/track6.rs:11:5
+  --> $DIR/track6.rs:LL:CC
    |
 LL |     default fn bar() {}
    |     ^^^^^^^^^^^^^^^^^^^
--Ztrack-diagnostics: created at $COMPILER_DIR/rustc_session/src/parse.rs:93:5
+-Ztrack-diagnostics: created at $COMPILER_DIR/rustc_session/src/parse.rs:LL:CC
    |
    = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
    = help: add `#![feature(specialization)]` to the crate attributes to enable