Add test for --remap-path-prefix on std imports

Author: cbeuw

src/test/codegen/remap_path_prefix/auxiliary/pseudo_std.rs

@@ -0,0 +1,8 @@
+// ignore-tidy-linelength
+
+// compile-flags: -g  --remap-path-prefix={{src-base}}/remap_path_prefix/auxiliary=/home/USERNAME/TOOLCHAIN/lib/rustlib/src/rust/library/std/src
+
+#[inline]
+pub fn some_std_function() -> i32 {
+    1234
+}

src/test/codegen/remap_path_prefix/issue-73167-remap-std.rs

@@ -0,0 +1,16 @@
+// ignore-windows
+// ignore-tidy-linelength
+
+// compile-flags: -g  -C no-prepopulate-passes --remap-path-prefix=/home/USERNAME=REDACTED
+// aux-build:pseudo_std.rs
+
+extern crate pseudo_std;
+// Here we check that imported code from std has their path remapped.
+// If we were to use an actual std function to test, we'd have to remap {{cwd}}/library/...,
+// but that will break if rustc was built with remap-debuginfo = true in config.toml.
+// Hence we simulate the path rust-src would take in a real linux installation
+// by remapping the prefix inside pseudo_std.rs
+// CHECK: !DIFile(filename: "REDACTED/TOOLCHAIN/lib/rustlib/src/rust/library/std/src/pseudo_std.rs", directory: ""
+fn main() {
+    pseudo_std::some_std_function();
+}