[FileCheck] Introduce substitution subclasses

Summary:
With now a clear distinction between string and numeric substitutions,
this patch introduces separate classes to represent them with a parent
class implementing the common interface. Diagnostics in
printSubstitutions() are also adapted to not require knowing which
substitution is being looked at since it does not hinder clarity and
makes the implementation simpler.

Reviewers: jhenderson, jdenny, probinson, arichardson

Subscribers: llvm-commits, probinson, arichardson, hiraditya

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62241

llvm-svn: 361446