Add cmake option to choose whether to use the builtin demangler

[labath]

Summary

Previously the builting demangler was on for platforms that explicitly set a flag by modifying
Mangled.cpp (windows, freebsd). The Xcode build always used builtin demangler by passing a
compiler flag. This adds a cmake flag (defaulting to ON) to configure the demangling library used
at build time. The flag is only available on non-windows platforms as there the system demangler
is not present (in the form we're trying to use it, at least).
The impact of this change is:

• linux: switches to the builtin demangler
• freebsd, windows: NFC (I hope)
• netbsd: switches to the builtin demangler
• osx cmake build: switches to the builtin demangler (matching the XCode build)

The main motivation for this is the cross-platform case, where it should bring more consistency
by removing the dependency on the host demangler (which can be completely unrelated to the debug
target).

[labath]

@krytarowski: I thought I'd switch netbsd as well, as all the other platforms are doing it already. However, if you want to stick with the system demangler by default, I can easily change it to OFF ...

[clayborg]

Looks good to me.

[krytarowski]

In D23830#524039, @labath wrote:

@krytarowski: I thought I'd switch netbsd as well, as all the other platforms are doing it already. However, if you want to stick with the system demangler by default, I can easily change it to OFF ...

Thanks, I need to take a closer look at it.

[krytarowski]

Looks OK for NetBSD. @joerg are you fine with this?

[joerg]

LGTM

cmake/modules/LLDBConfig.cmake

if (NOT LLDB_USING_LIBSTDCXX_4_9 AND NOT LLVM_ENABLE_EH)
             "lldb crashes. See <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59656> for "
             "details. Possible courses of action are:\n"
             "- use libstdc++ version 4.9 or newer\n"
             "- use libc++ (via LLVM_ENABLE_LIBCXX)\n"
             "- enable exceptions (via LLVM_ENABLE_EH)\n"
             "- ignore this warning and accept occasional instability")
     endif()
 endif()
+
+if(MSVC)
+    set(LLDB_USE_BUILTIN_DEMANGLER ON)
+else()
+    option(LLDB_USE_BUILTIN_DEMANGLER "Use lldb's builtin demangler instead of the system one" ON)
+endif()
+if(LLDB_USE_BUILTIN_DEMANGLER)
+    add_definitions(-DLLDB_USE_BUILTIN_DEMANGLER)
+endif()

source/Core/Mangled.cpp

 //===-- Mangled.cpp ---------------------------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 
 // FreeBSD9-STABLE requires this to know about size_t in cxxabi.h
 #include <cstddef>
 #if defined(_MSC_VER)
 #include "lldb/Host/windows/windows.h"
 #include <Dbghelp.h>
 #pragma comment(lib, "dbghelp.lib")
-#define LLDB_USE_BUILTIN_DEMANGLER
-#elif defined (__FreeBSD__)
-#define LLDB_USE_BUILTIN_DEMANGLER
-#else
-#include <cxxabi.h>
 #endif
 
 #ifdef LLDB_USE_BUILTIN_DEMANGLER
-
 // Provide a fast-path demangler implemented in FastDemangle.cpp until it can
 // replace the existing C++ demangler with a complete implementation
 #include "lldb/Core/FastDemangle.h"
 #include "lldb/Core/CxaDemangle.h"
-
+#else
+#include <cxxabi.h>
 #endif
 
 
 #include "llvm/ADT/DenseMap.h"
 
 #include "lldb/Core/ConstString.h"
 #include "lldb/Core/Log.h"
 #include "lldb/Core/Logging.h"