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

[StephanTLavavej]

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:

STL/tests/std/tests/P0881R7_stacktrace/test.cpp

Line 193 in defc75b

assert(trim_past_plus(all.at(0).description()) == "P0881R7_stacktrace!all_innermost"sv);

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:

STL/tests/std/tests/P0881R7_stacktrace/test.cpp

Lines 103 to 108 in defc75b

	string trim_past_plus(string str) {
	if (size_t pos = str.rfind("+", string::npos); pos != string::npos) {
	str.resize(pos);
	}
	return str;
	}

Similarly, in the header units and modules test,

STL/tests/std/include/test_header_units_and_modules.hpp

Lines 851 to 861 in defc75b

	auto desc = stacktrace::current().at(0).description();
	if (auto pos = desc.find("!"); pos != string::npos) {
	desc = desc.substr(pos + 1);
	}
	if (auto pos = desc.find("+"); pos != string::npos) {
	desc.resize(pos);
	}
	assert(desc == "test_stacktrace");

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.