Fix MemoryBuffer::getSTDIN to *not* return null if stdin is empty, this is a lame API.

Also, Stringrefify some more MemoryBuffer functions, and add two performance FIXMEs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86630 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed