[tools] Show helpful message when demangling '_' (#61369)

From most shells, the symbol(s) given to `swift demangle` should be quoted or escaped, otherwise the shell will perform variable expansion on the `$...` suffix of the symbol. For example:

```
% swift demangle _$sScMMa
_ ---> _
```

Instead the command should be edited to one of the following:
```
% swift demangle sScMMa
_$sScMMa ---> type metadata accessor for Swift.MainActor
% swift demangle '_$sScMMa'
_$sScMMa ---> type metadata accessor for Swift.MainActor
% swift demangle _\$sScMMa
_$sScMMa ---> type metadata accessor for Swift.MainActor
```

When variable expansion has happened, `swift demangle` will see the symbol only as "`_`". This change adds a warning that identifies the likely issue, and contains suggestions on how the user can fix and rerun the invocation.

Author: kastiglione

tools/swift-demangle/swift-demangle.cpp

@@ -391,6 +391,12 @@ int main(int argc, char **argv) {
   } else {
     swift::Demangle::Context DCtx;
     for (llvm::StringRef name : InputNames) {
+      if (name == "_") {
+        llvm::errs() << "warning: input symbol '_' is likely the result of "
+                        "variable expansion by the shell. Ensure the argument "
+                        "is quoted or escaped.\n";
+        continue;
+      }
       if (name.startswith("S") || name.startswith("s") ) {
         std::string correctedName = std::string("$") + name.str();
         demangle(llvm::outs(), correctedName, DCtx, options);