`<stacktrace>`: ARM64EC isn't working properly

While working on enabling ARM64EC runtime test coverage (on an ARM64 host), I'm observing that <stacktrace> doesn't work.

For example, we expect "P0881R7_stacktrace!all_innermost" here:

https://github.com/microsoft/STL/blob/defc75b4758bd03e075dc9d0de093572dc57c160/tests/std/tests/P0881R7_stacktrace/test.cpp#L193

This works for x64, x86, and ARM64. But for ARM64EC, we get "P0881R7_stacktrace". The content of all.at(0).description() is "P0881R7_stacktrace+0xB0E0" and the "+0xB0E0" is being trimmed off:

https://github.com/microsoft/STL/blob/defc75b4758bd03e075dc9d0de093572dc57c160/tests/std/tests/P0881R7_stacktrace/test.cpp#L103-L108

Similarly, in the header units and modules test,

https://github.com/microsoft/STL/blob/defc75b4758bd03e075dc9d0de093572dc57c160/tests/std/include/test_header_units_and_modules.hpp#L851-L861

desc is "P2465R3_standard_library_modules+0x95254" before trimming and "P2465R3_standard_library_modules" after.

My understanding of ARM64EC is extremely limited, so I don't know if this is an issue in dbgeng.dll, or the debug info that the compiler is emitting, or what.

Start with tests/std/tests/P0881R7_stacktrace/test.cpp, especially the expected innermost name and description trimming, then compare tests/std/include/test_header_units_and_modules.hpp. Investigate whether ARM64EC symbol data or dbgeng.dll causes the offset to be removed; done means the relevant ARM64EC tests preserve the expected function names and pass.