Improve error message of FileCheck when stdin is empty

Summary: Replace '-' in the error message with <stdin>. This is also consistent with another error message in the code.

Reviewers: jhenderson, probinson, jdenny, grimar, arichardson

Reviewed By: jhenderson

Subscribers: thopre, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73793