[lldb] Let Mangled decide whether a name is mangled or not

[lldb] Let Mangled decide whether a name is mangled or not

We have a handful of places in LLDB where we try to outsmart the logic
in Mangled to determine whether a string is mangled or not. There's at
least one place (*) where we are getting this wrong and causes a subtle
bug. The `cstring_is_mangled` is cheap enough that we should always rely
on it to determine whether a string is mangled or not.

(*) `ObjectFileMachO` assumes that a symbol that starts with a double
underscore (such as `__pthread_kill`) is mangled. That's mostly
harmless, until you use `function.name-without-args` in the frame
format. The formatter calls `Symbol::GetNameNoArguments()` which is a
wrapper around `Mangled::GetName(ePreferDemangledWithoutArguments)`. The
latter will first try using the appropriate language plugin to get the
demangled name without arguments, and if that fails, falls back to
returning the demangled name. Because we forced Mangled to treat the
symbol as a mangled name (even though it's not) there's no demangled
name. The result is that frames don't show any symbol at all.

Differential revision: https://reviews.llvm.org/D148846