Fix merging sourcemaps on Windows (#14282)

packages/babel-core/src/transformation/file/merge-map.ts

@@ -4,8 +4,16 @@ import remapping from "@ampproject/remapping";
 export default function mergeSourceMap(
   inputMap: SourceMap,
   map: SourceMap,
-  source: string,
+  sourceFileName: string,
 ): SourceMap {
+  // On win32 machines, the sourceFileName uses backslash paths like
+  // `C:\foo\bar.js`. But sourcemaps are always posix paths, so we need to
+  // normalize to regular slashes before we can merge (else we won't find the
+  // source associated with our input map).
+  // This mirrors code done while generating the output map at
+  // https://github.com/babel/babel/blob/5c2fcadc9ae34fd20dd72b1111d5cf50476d700d/packages/babel-generator/src/source-map.ts#L102
+  const source = sourceFileName.replace(/\\/g, "/");
+
   // Prevent an infinite recursion if one of the input map's sources has the
   // same resolved path as the input map. In the case, it would keep find the
   // input map, then get it's sources which will include a path like the input