Format: ArrayRefize some implicit copies away.

NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219000 91177308-0d34-0410-b5e6-96231b3b80d8
ASTUnit: Replace out parameter with return value, we have NRVO for this.

And elide the copy when pushing the return value into a vector. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219001 91177308-0d34-0410-b5e6-96231b3b80d8