Add a -D flag to FileCheck to define variables

[arichardson]

Summary

This makes it very easy to test files that only differ in a constant
value somewhere in the test case.

[probinson]

docs/CommandGuide/FileCheck.rst

89

Is there an extra = here? I'm a little unclear on the details of how this markup works.

[arichardson]

docs/CommandGuide/FileCheck.rst

89

Ah yes I think there is. Will fix before committing

[probinson]

It does seem like this could be very useful in a sequence of RUN commands, where previously we'd have to add a bunch of run-specific checks.
LGTM.

docs/CommandGuide/FileCheck.rst

.. option:: --enable-var-scope
 
   Enables scope for regex variables.
 
   Variables with names that start with ``$`` are considered global and
   remain set throughout the file.
 
   All other variables get undefined after each encountered ``CHECK-LABEL``.
 
+.. option:: -D=<VAR=VALUE>
+
+  Sets a filecheck variable ``VAR`` with value ``VALUE`` that can be used in
+  ``CHECK:`` lines.
+
 .. option:: -version
 
  Show the version number of this program.
 
 EXIT STATUS
 -----------
 
 If :program:`FileCheck` verifies that the file matches the expected contents,

test/FileCheck/defines.txt

+; RUN: FileCheck -DVALUE=10 -input-file %s %s
+; RUN: not FileCheck -DVALUE=20 -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRMSG
+
+Value = 10
+; CHECK: Value = [[VALUE]]
+
+; ERRMSG: defines.txt:5:10: error: expected string not found in input
+; ERRMSG: defines.txt:1:1: note: with variable "VALUE" equal to "20"
+; ERRMSG: defines.txt:4:1: note: possible intended match here

utils/FileCheck/FileCheck.cpp

 
 static cl::list<std::string> ImplicitCheckNot(
     "implicit-check-not",
     cl::desc("Add an implicit negative check with this pattern to every\n"
              "positive check. This can be used to ensure that no instances of\n"
              "this pattern occur which are not matched by a positive pattern"),
     cl::value_desc("pattern"));
 
+static cl::list<std::string> GlobalDefines("D", cl::Prefix,
+    cl::desc("Define a variable to be used in capture patterns."),
+    cl::value_desc("VAR=VALUE"));
+
 static cl::opt<bool> AllowEmptyInput(
     "allow-empty", cl::init(false),
     cl::desc("Allow the input file to be empty. This is useful when making\n"
              "checks that some error message does not occur, for example."));
 
 static cl::opt<bool> MatchFullLines(
     "match-full-lines", cl::init(false),
     cl::desc("Require all positive matches to cover an entire input line.\n"

 /// Returns false if the input fails to satisfy the checks.
 bool CheckInput(SourceMgr &SM, StringRef Buffer,
                 ArrayRef<CheckString> CheckStrings) {
   bool ChecksFailed = false;
 
   /// VariableTable - This holds all the current filecheck variables.
   StringMap<StringRef> VariableTable;
 
+  for (const auto& Def : GlobalDefines)
+    VariableTable.insert(StringRef(Def).split('='));
+
   unsigned i = 0, j = 0, e = CheckStrings.size();
   while (true) {
     StringRef CheckRegion;
     if (j == e) {
       CheckRegion = Buffer;
     } else {
       const CheckString &CheckLabelStr = CheckStrings[j];
       if (CheckLabelStr.Pat.getCheckTy() != Check::CheckLabel) {