[FileCheck] Fix parsing empty global and pseudo variable names (#83667)

Reland #82595 with fixes of build failures related to colored output.
See https://lab.llvm.org/buildbot/#/builders/139/builds/60549
Use `%ProtectFileCheckOutput` to avoid colored output.
Original commit message below.

In `Pattern::parseVariable`, for global variables (those starting with '$')
and for pseudo variables (those starting with '@') the first character is
consumed before actual variable name parsing. If the name is empty, it
leads to out-of-bound access to the corresponding `StringRef`.

This patch adds an if statement against the case described.

Author: kovdan01

llvm/lib/FileCheck/FileCheck.cpp

@@ -297,6 +297,12 @@ Pattern::parseVariable(StringRef &Str, const SourceMgr &SM) {
   if (Str[0] == '$' || IsPseudo)
     ++I;
 
+  if (I == Str.size())
+    return ErrorDiagnostic::get(SM, Str.slice(I, StringRef::npos),
+                                StringRef("empty ") +
+                                    (IsPseudo ? "pseudo " : "global ") +
+                                    "variable name");
+
   if (!isValidVarNameStart(Str[I++]))
     return ErrorDiagnostic::get(SM, Str, "invalid variable name");
 

llvm/test/FileCheck/empty-variable-name.txt

@@ -0,0 +1,32 @@
+a
+
+; RUN: %ProtectFileCheckOutput not FileCheck -input-file %s %s 2>&1 | \
+; RUN:   FileCheck -check-prefix CHECK-ERROR -DDIR=%S \
+; RUN:   --match-full-lines --strict-whitespace %s
+
+; CHECK:        a[[]]
+; CHECK-ERROR:[[DIR]]{{/|\\}}empty-variable-name.txt:7:13: error: empty variable name
+; CHECK-ERROR-NEXT:; CHECK: a{{\[\[\]\]}}
+;      CHECK-ERROR-NEXT:            ^
+
+b
+
+; RUN: %ProtectFileCheckOutput not FileCheck -input-file %s -check-prefix CHECK-PSEUDO %s 2>&1 | \
+; RUN:   FileCheck -check-prefix CHECK-ERROR-PSEUDO -DDIR=%S \
+; RUN:   --match-full-lines --strict-whitespace %s
+
+; CHECK-PSEUDO: b[[@]]
+; CHECK-ERROR-PSEUDO:[[DIR]]{{/|\\}}empty-variable-name.txt:18:21: error: empty pseudo variable name
+; CHECK-ERROR-PSEUDO-NEXT:; CHECK-PSEUDO: b{{\[\[@\]\]}}
+;      CHECK-ERROR-PSEUDO-NEXT:                    ^
+
+c
+
+; RUN: %ProtectFileCheckOutput not FileCheck -input-file %s -check-prefix CHECK-GLOBAL %s 2>&1 | \
+; RUN:   FileCheck -check-prefix CHECK-ERROR-GLOBAL -DDIR=%S \
+; RUN:   --match-full-lines --strict-whitespace %s
+
+; CHECK-GLOBAL: c[[$]]
+; CHECK-ERROR-GLOBAL:[[DIR]]{{/|\\}}empty-variable-name.txt:29:21: error: empty global variable name
+; CHECK-ERROR-GLOBAL-NEXT:; CHECK-GLOBAL: c{{\[\[\$\]\]}}
+;       CHECK-ERROR-GLOBAL-NEXT:                    ^