[FileCheck] Better diagnostic for format conflict

Summary:
Improve error message in case of conflict between several implicit
format to mention the operand that conflict.

Reviewers: jhenderson, jdenny, probinson, grimar, arichardson, rnk

Reviewed By: jdenny

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77741