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
2a96d19ab0