[test] FileCheck-ify a test to avoid a spurious failure, NFC

The path to one of my source trees contains 'builtin' as a substring, so
this test failed. Fix it with FileCheck.

llvm-svn: 309460

clang/test/CodeGen/2004-02-20-Builtins.c

@@ -1,5 +1,8 @@
-// RUN: %clang_cc1  %s -emit-llvm -o - | not grep builtin
+// RUN: %clang_cc1  %s -emit-llvm -o - | FileCheck %s
 double sqrt(double x);
+
+// CHECK-LABEL: @zsqrtxxx
+// CHECK-NOT: builtin
 void zsqrtxxx(float num) {
    num = sqrt(num);
 }