[NFC][FileCheck] Add --allow-unused-prefixes option

[mtrofin]

Summary

This introduces the option and plumbs it through, but does not leverage
it just yet.

The background is: FileCheck only checks that one prefix is used, so if
more are specified, some may never be exercised. This may be intentional
or accidental. We want to catch the latter. Currently there are ~1500
tests exhibiting this. The option allows tests to opt in to the behavior
(if that is intended), following which, we enable strict prefix checking
(D90281)

[MaskRay]

Emm... I think you can just reuse D90281 as we want the option to come with its functionality to test that the option is actually useful.

llvm/utils/FileCheck/FileCheck.cpp

80

true by default then switch to false by default

[mtrofin]

Merged with D90281

llvm/include/llvm/FileCheck/FileCheck.h

 /// Contains info about various FileCheck options.
 struct FileCheckRequest {
   std::vector<StringRef> CheckPrefixes;
   std::vector<StringRef> CommentPrefixes;
   bool NoCanonicalizeWhiteSpace = false;
   std::vector<StringRef> ImplicitCheckNot;
   std::vector<StringRef> GlobalDefines;
   bool AllowEmptyInput = false;
+  bool AllowUnusedPrefixes = false;
   bool MatchFullLines = false;
   bool IgnoreCase = false;
   bool IsDefaultCheckPrefix = false;
   bool EnableVarScope = false;
   bool AllowDeprecatedDagOverlap = false;
   bool Verbose = false;
   bool VerboseVerbose = false;
 };

llvm/utils/FileCheck/FileCheck.cpp

GlobalDefines("D", cl::AlwaysPrefix,
                   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> AllowUnusedPrefixes(
+    "allow-unused-prefixes", cl::init(false),
+    cl::desc("Allow prefixes to be specified but not appear in the test."));
+
 static cl::opt<bool> MatchFullLines(
     "match-full-lines", cl::init(false),
     cl::desc("Require all positive matches to cover an entire input line.\n"
              "Allows leading and trailing whitespace if --strict-whitespace\n"
              "is not also passed."));
 
 static cl::opt<bool> EnableVarScope(
     "enable-var-scope", cl::init(false),
if (EqIdx == 0) {
       continue;
     }
     Req.GlobalDefines.push_back(G);
   }
   if (GlobalDefineError)
     return 2;
 
   Req.AllowEmptyInput = AllowEmptyInput;
+  Req.AllowUnusedPrefixes = AllowUnusedPrefixes;
   Req.EnableVarScope = EnableVarScope;
   Req.AllowDeprecatedDagOverlap = AllowDeprecatedDagOverlap;
   Req.Verbose = Verbose;
   Req.VerboseVerbose = VerboseVerbose;
   Req.NoCanonicalizeWhiteSpace = NoCanonicalizeWhiteSpace;
   Req.MatchFullLines = MatchFullLines;
   Req.IgnoreCase = IgnoreCase;