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.

llvm-svn: 67448

clang/test/Frontend/stdin.c

+// RUN: clang -E - < /dev/null &> %t
+// RUN: grep '<predefines>' %t
+