[lldb] Fix PR52702 by fixing bool conversion of Mangled

Remove the Mangled::operator! and Mangled::operator void* where the
comments in header and implementation files disagree and replace them
with operator bool.

This fix PR52702 as https://reviews.llvm.org/D106837 used the buggy
Mangled::operator! in Symbol::SynthesizeNameIfNeeded. For example,
consider the symbol "puts" in a hello world C program:

// Inside Symbol::SynthesizeNameIfNeeded
(lldb) p m_mangled
(lldb_private::Mangled) $0 = (m_mangled = None, m_demangled = "puts")
(lldb) p !m_mangled
(bool) $1 = true          # should be false!!
This leads to Symbol::SynthesizeNameIfNeeded overwriting m_demangled
part of Mangled (in this case "puts").

In conclusion, this patch turns
callq  0x401030                  ; symbol stub for: ___lldb_unnamed_symbol36
back into
callq  0x401030                  ; symbol stub for: puts .

Differential Revision: https://reviews.llvm.org/D116217