[lldb][NFC] Update syntax description for language cplusplus demangle

[bulbazord]

Summary

Also added some tests because this is completely untested.

rdar://107780577

lldb/source/Plugins/LanguageRuntime/CPlusPlus/ItaniumABI/ItaniumABILanguageRuntime.cpp

if (language == eLanguageTypeC_plus_plus ||
     return new ItaniumABILanguageRuntime(process);
   else
     return nullptr;
 }
 
 class CommandObjectMultiwordItaniumABI_Demangle : public CommandObjectParsed {
 public:
   CommandObjectMultiwordItaniumABI_Demangle(CommandInterpreter &interpreter)
-      : CommandObjectParsed(interpreter, "demangle",
-                            "Demangle a C++ mangled name.",
-                            "language cplusplus demangle") {
+      : CommandObjectParsed(
+            interpreter, "demangle", "Demangle a C++ mangled name.",
+            "language cplusplus demangle [<mangled-name> ...]") {
     CommandArgumentEntry arg;
     CommandArgumentData index_arg;
 
     // Define the first (and only) variant of this arg.
     index_arg.arg_type = eArgTypeSymbol;
     index_arg.arg_repetition = eArgRepeatPlus;
 
     // There is only one variant this argument could be; put it into the

lldb/test/Shell/Commands/command-language-cplusplus-demangle.test

+# RUN: %lldb -b -o "language cplusplus demangle __ZN3Foo7DoThingEv" \
+# RUN:   | FileCheck --check-prefix=DOUBLE-UNDERSCORE %s
+# RUN: %lldb -b -o "language cplusplus demangle _ZN3Foo7DoThingEv" \
+# RUN:   | FileCheck --check-prefix=SINGLE-UNDERSCORE %s
+# RUN: not %lldb -b -o "language cplusplus demangle foo" 2>&1 \
+# RUN:   | FileCheck --check-prefix=NOT-MANGLED %s
+# RUN: not %lldb -b -o "language cplusplus demangle _ZN3Foo7DoThingEv foo" 2>&1 \
+# RUN:   | FileCheck --check-prefix=MULTI-ARG %s
+# RUN: %lldb -b -o "help language cplusplus demangle" \
+# RUN:   | FileCheck --check-prefix=HELP-MESSAGE %s
+
+# DOUBLE-UNDERSCORE: __ZN3Foo7DoThingEv ---> Foo::DoThing()
+
+# SINGLE-UNDERSCORE: _ZN3Foo7DoThingEv ---> Foo::DoThing()
+
+# NOT-MANGLED: error: foo is not a valid C++ mangled name
+
+# MULTI-ARG: _ZN3Foo7DoThingEv ---> Foo::DoThing()
+# MULTI-ARG: error: foo is not a valid C++ mangled name
+
+# HELP-MESSAGE: Demangle a C++ mangled name.
+# HELP-MESSAGE: Syntax: language cplusplus demangle [<mangled-name> ...]