[OSX] lldb cannot read bitfield dwarf debug info generated by -mmacosx-version-min=10.11

Type: Debugger

Describe the bug

• OS and Version: MacOSX 10.4.1
• VS Code Version: 1.29.0
• C/C++ Extension Version: 0.20.1
• A clear and concise description of what the bug is.

Starting from MacOSX target 10.11, clang tunes it's debug information for lldb. This causes the version of lldb embedded in the cpptools extension to misreport bitfield values.

To Reproduce

#include <stdio.h>

int main() {
    struct Test {
        unsigned short a;
        unsigned short b : 1;
        unsigned short c: 15;
    };

    Test x = {0};
    x.a = 5;
    printf("%hu", x.b);
    return 0;
}

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "(lldb) Launch",
            "type": "cppdbg",
            "request": "launch",
            "program": "${workspaceFolder}/a.out",
            "args": [],
            "stopAtEntry": false,
            "cwd": "${workspaceFolder}",
            "environment": [],
            "externalConsole": true,
            "MIMode": "lldb"
        }
    ]
}

Steps to reproduce the behavior:

1. Run clang++ test.cc -mmacosx-version-min=10.11 -g
2. Place a breakpoint at line 10: x.a = 5
3. Launch debugger
4. Step over x.a = 5 and see that x.b or x.c changes value (lldb cannot find the right addresses for the bitfields.

Additional context
Compilation where older lldb works:

1. clang++ -c test.cc -mmacosx-version-min=10.10 -g
2. clang++ -c test.cc -mmacosx-version-min=10.11 -ggdb

Start by reproducing the minimal test.cc case with clang++ using -mmacosx-version-min=10.10 and 10.11, then launch it with the provided .vscode/launch.json configuration. Compare how the debugger reports x.b and x.c after stepping over x.a = 5; done means bitfield values remain correct for the 10.11 target.