FileCheck: Improve FileCheck variable terminology

FileCheck: Improve FileCheck variable terminology

Summary:
Terminology introduced by [[#]] blocks is confusing and does not
integrate well with existing terminology.

First, variables referred by [[]] blocks are called "pattern variables"
while the text a CHECK directive needs to match is called a "CHECK
pattern". This is inconsistent with variables in [[#]] blocks since
[[#]] blocks are also found in CHECK pattern yet those variables are
called "numeric variable".

Second, the replacing of both [[]] and [[#]] blocks by the value of the
variable or expression they contain is represented by a
FileCheckPatternSubstitution class. The naming refers to being a
substitution in a CHECK pattern but could be wrongly understood as being
a substitution of a pattern variable.

Third and lastly, comments use "numeric expression" to refer both to the
[[#]] blocks as well as to the numeric expressions these blocks contain
which get evaluated at match time.

This patch solves these confusions by
- calling variables in [[]] and [[#]] blocks as string and numeric
  variables respectively;
- referring to [[]] and [[#]] as substitution *blocks*, with the former
  being a string substitution block and the latter a numeric
  substitution block;
- calling [[]] and [[#]] blocks to be replaced by the value of a
  variable or expression they contain a substitution (as opposed to
  definition when these blocks are used to defined a variable), with the
  former being a string substitution and the latter a numeric
  substitution;
- renaming the FileCheckPatternSubstitution as a FileCheckSubstitution
  class with FileCheckStringSubstitution and
  FileCheckNumericSubstitution subclasses;
- restricting the use of "numeric expression" to refer to the expression
  that is evaluated in a numeric substitution.

While numeric substitution blocks only support numeric substitutions of
numeric expressions at the moment there are plans to augment numeric
substitution blocks to support numeric definitions as well as both a
numeric definition and numeric substitution in the same numeric
substitution block.

Reviewers: jhenderson, jdenny, probinson, arichardson

Subscribers: hiraditya, arichardson, probinson, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62146

llvm-svn: 361445