Enable the ARM Neon intrinsics test by default.

The test being marked 'REQUIRES: long-tests' doesn't make sense. It's not the
first time the test is broken without being noticed by the committer. If the
test is too long, it should be shortened, split in multiple ones or removed
altogether. Keeping it as is is actively harmful.
(BTW, on my machine `ninja check-clang` takes 90-92 seconds with and without
this test. The difference in times is below the spread caused by random
factors.)

llvm-svn: 304302