test: loosen -Xllvm version string matching

If the LLVM/clang is built with release mode and PACKAGE_VENDOR set, it
will not print the same string but rather the version number.  Permit
either string.

Author: compnerd

test/Frontend/Xllvm.swift

@@ -2,4 +2,4 @@
 // CHECK-HELP: -version
 
 // RUN: %swift -Xllvm -version -emit-sil %s 2>&1 | %FileCheck %s -check-prefix=CHECK-SIL
-// CHECK-SIL: LLVM (http://llvm.org/)
+// CHECK-SIL: LLVM {{(http://llvm.org/)|version }}