Natvis format specifier `,s` doesn't work

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Issue Type: Bug

Format specifier ,s doesn't work.

How to reproduce

Compile and debug the following code:
(Mingw gcc 11 is used)

#include <string>

int main()
{
    std::string string("test");

    return 0;
}

Use the following natvis entity:

    <Type Name="std::string">
        <AlternativeType Name="std::__cxx11::basic_string&lt;*&gt;"/>
        <DisplayString>{_M_dataplus._M_p,s}</DisplayString>
        <Expand>
            <Item Name="size">_M_string_length</Item>
            <Item Name="capacity" Condition="_M_dataplus._M_p == _M_local_buf">sizeof(_M_local_buf)</Item>
            <Item Name="capacity" Condition="_M_dataplus._M_p != _M_local_buf">_M_allocated_capacity</Item>
            <Item Name="string">_M_dataplus._M_p,s</Item>
        </Expand>
    </Type>

See that ,s format specifier in <DisplayString> and <Item Name="string"> doesn't change anything in debug view, hover and showDisplayString.

Actual result:

Expected result: string = "test" (without leading address)

Extension version: 1.5.0-insiders2
VS Code version: Code 1.57.1 (507ce72a4466fbb27b715c3722558bb15afa9f48, 2021-06-17T13:28:07.755Z)
OS version: Windows_NT x64 10.0.17763
Restricted Mode: No

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz (8 x 1992)
GPU Status	2d_canvas: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on oop_rasterization: enabled opengl: enabled_on rasterization: enabled skia_renderer: enabled_on video_decode: enabled vulkan: disabled_off webgl: enabled webgl2: enabled
Load (avg)	undefined
Memory (System)	15.85GB (8.96GB free)
Process Argv	--locale=en --crash-reporter-id c8510ba5-736a-4465-9c29-9471122a68c5
Screen Reader	no
VM	0%

A/B Experiments

vsliv368:30146709
vsreu685:30147344
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pythonvspyt602:30300191
vspor879:30202332
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vspor363:30204092
pythonvspyt639:30300192
pythontb:30283811
pythonvspyt551cf:30311713
vspre833:30321513
pythonptprofiler:30281270
vshan820:30294714
pythondataviewer:30285071
vscus158:30321503
pythonvsuse255cf:30323309
vscorehov:30309549
vscod805cf:30301675
pythonvspyt200:30324779
binariesv615:30325510

No source file or test is named in the issue. Reproduce the behavior with the supplied C++ program and Natvis entity, then trace handling of the ,s format specifier in DisplayString and Item expressions. Done means the debug view, hover, and showDisplayString render string = "test" without the leading address.

Written by the indexing model from the issue text.

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