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