cmd/compile: improved error message when calling a shadowed builtin

Otherwise, the error can be confusing if one forgets or doesn't know
that the builtin is being shadowed, which is not common practice.

Fixes #22822.

Change-Id: I735393b5ce28cb83815a1c3f7cd2e7bb5080a32d
Reviewed-on: https://go-review.googlesource.com/97455
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/universe.go

@@ -65,6 +65,17 @@ var builtinFuncs = [...]struct {
 	{"recover", ORECOVER},
 }
 
+// isBuiltinFuncName reports whether name matches a builtin function
+// name.
+func isBuiltinFuncName(name string) bool {
+	for _, fn := range builtinFuncs {
+		if fn.name == name {
+			return true
+		}
+	}
+	return false
+}
+
 var unsafeFuncs = [...]struct {
 	name string
 	op   Op