Normalize paths in diagnostics CLI output (#4709)

Summary:
This should fix the current breakage of our windows tests. One thing I'm not sure about is if this will cause any problems with things like clickable file paths in the terminal on Windows.

Pull Request resolved: #4709

Test Plan: rust tests pass on Windows GitHub CI https://github.com/facebook/relay/actions/runs/9420850672/job/25953760675?pr=4709

Reviewed By: tyao1

Differential Revision: D58296386

Pulled By: captbaritone

fbshipit-source-id: 0b2c731127f1063731a51ea23fe9898d0b46cfa2

compiler/crates/graphql-cli/src/diagnostic_printer.rs

@@ -85,7 +85,7 @@ impl<TSources: Sources> DiagnosticPrinter<TSources> {
             writeln!(
                 writer,
                 "  {}{}",
-                location.source_location().path().underline(),
+                normalize_path(location.source_location().path()).underline(),
                 format!(":{}:{}", range.start.line + 1, range.start.character + 1).dimmed()
             )?;
             source_printer.write_span_with_highlight_style(
@@ -99,7 +99,7 @@ impl<TSources: Sources> DiagnosticPrinter<TSources> {
             writeln!(
                 writer,
                 "{}: <missing source>",
-                location.source_location().path()
+                normalize_path(location.source_location().path())
             )?;
         }
         Ok(())
@@ -118,3 +118,9 @@ where
         self(source_location)
     }
 }
+
+/// Normalize Windows paths to Unix style. This is important for stable test
+/// output across Mac/Windows/Linux.
+fn normalize_path(path: &str) -> String {
+    path.replace("\\", "/")
+}