[PATCH] Fix the prefix for arm64 triple

[Yi Kong]

>>! In D4516#9, @kongyi wrote:
>>>! In D4516#8, @t.p.northover wrote:
>> It looks like it's only used by that part of Clang. We probably want a test though (even if it's in Clang rather than LLVM), since this is clearly causing problems for someone.
> 
> I have written a test case in Clang, however due to the limitation of Phabricator, I can't post it in the same post.

Actually, it's part of D4520. The test makes sure that GCCBuiltin works when the triple supplied is ARM64, indicating LLVM returned the correct prefix.

> I will take another look to see whether it is possible to test in LLVM.

I can't find a way to do that. It seems we can only test this indirectly from Clang.

http://reviews.llvm.org/D4516