[lldb][CPlusPlusLanguage] Avoid redundant const char* -> StringRef roundtrip (llvm#161499)

We've been seen (very sporadic) lifetime issues around this area. Here's
an example backtrace:
```
[  8] 0x0000000188e56743 libsystem_platform.dylib`_sigtramp + 55
[  9] 0x00000001181e041f LLDB`lldb_private::CPlusPlusLanguage::SymbolNameFitsToLanguage(lldb_private::Mangled) const [inlined] unsigned long std::1::constexpr_strlen[abi:nn200100]<char>(char const*) + 7 at constexpr_c_functions.h:63:10
[  9] 0x00000001181e0418 LLDB`lldb_private::CPlusPlusLanguage::SymbolNameFitsToLanguage(lldb_private::Mangled) const [inlined] std::__1::char_traits<char>::length[abi:nn200100](char const*) at char_traits.h:232:12
[  9] 0x00000001181e0418 LLDB`lldb_private::CPlusPlusLanguage::SymbolNameFitsToLanguage(lldb_private::Mangled) const [inlined] llvm::StringRef::StringRef(char const*) at StringRef.h:90:33
[  9] 0x00000001181e0418 LLDB`lldb_private::CPlusPlusLanguage::SymbolNameFitsToLanguage(lldb_private::Mangled) const [inlined] llvm::StringRef::StringRef(char const*) at StringRef.h:92:38
[  9] 0x00000001181e0418 LLDB`lldb_private::CPlusPlusLanguage::SymbolNameFitsToLanguage(lldb_private::Mangled) const + 20 at CPlusPlusLanguage.cpp:68:62
```

Looks like we're calling `strlen` on a nullptr. I stared at this
codepath for a while but am still not sure how that could happen unless
the underlying `ConstString` somehow pointed to corrupted data.

But `SymbolNameFitsToLanguage` does some roundtripping through a `const
char*` before calling `GetManglingScheme`. No other callsite does this
and it just seems redundant.

This patch cleans this up.

rdar://161128180

Author: Michael137

lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp

@@ -104,10 +104,10 @@ CPlusPlusLanguage::GetFunctionNameInfo(ConstString name) const {
 }
 
 bool CPlusPlusLanguage::SymbolNameFitsToLanguage(Mangled mangled) const {
-  const char *mangled_name = mangled.GetMangledName().GetCString();
-  auto mangling_scheme = Mangled::GetManglingScheme(mangled_name);
-  return mangled_name && (mangling_scheme == Mangled::eManglingSchemeItanium ||
-                          mangling_scheme == Mangled::eManglingSchemeMSVC);
+  auto mangling_scheme =
+      Mangled::GetManglingScheme(mangled.GetMangledName().GetStringRef());
+  return mangling_scheme == Mangled::eManglingSchemeItanium ||
+         mangling_scheme == Mangled::eManglingSchemeMSVC;
 }
 
 ConstString CPlusPlusLanguage::GetDemangledFunctionNameWithoutArguments(