Frontend: Handle empty input on stdin.

Frontend: Handle empty input on stdin.
 - PR3854.

I think it makes more sense to change MemoryBuffer::getSTDIN (return 0
should indicate error, not empty), but it is documented to return 0
for empty inputs, and some other code appears to rely on this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67449 91177308-0d34-0410-b5e6-96231b3b80d8