Replace a lookup_debug_loc call.

`lookup_debug_loc` finds a file, line, and column, which requires two
binary searches. But this call site only needs the file.

This commit replaces the call with `lookup_source_file`, which does a
single binary search.

Author: nnethercote

compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs

@@ -65,10 +65,10 @@ fn make_mir_scope<'ll, 'tcx>(
         debug_context.scopes[parent]
     } else {
         // The root is the function itself.
-        let loc = cx.lookup_debug_loc(mir.span.lo());
+        let file = cx.sess().source_map().lookup_source_file(mir.span.lo());
         debug_context.scopes[scope] = DebugScope {
-            file_start_pos: loc.file.start_pos,
-            file_end_pos: loc.file.end_pos,
+            file_start_pos: file.start_pos,
+            file_end_pos: file.end_pos,
             ..debug_context.scopes[scope]
         };
         instantiated.insert(scope);