clang-format: Allow unbroken ::: in inline assembly.

Before:
  asm volatile("nop" :: : "memory");

After:
  asm volatile("nop" ::: "memory");

Patch by Eugene Toder. Thank you.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217883 91177308-0d34-0410-b5e6-96231b3b80d8