[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.

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/FileCheck/FileCheck.cpp b/llvm/lib/FileCheck/FileCheck.cpp
index 6d3a2b9..8f80a69 100644
--- a/llvm/lib/FileCheck/FileCheck.cpp
+++ b/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");