[FileCheck] Remove FileCheck prefix in API

Summary:
When FileCheck was made a library, types in the public API were renamed
to add a FileCheck prefix, such as Pattern to FileCheckPattern. Many
types were moved into a private interface and thus don't need this
prefix anymore. This commit removes those unneeded prefixes.

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

Reviewed By: jhenderson

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72186