Add _bstr user-defined literal for BSTR-shaped wide strings

[jonwis]

Reduce global initializers and intermediate allocations with a BSTR-compatible string literal.

// Unnecessary globals - 
const wil::unique_bstr g_bst(SysAllocString(L"kittens"));
const auto g_bst2 { wil::make_bstr_failfast(L"puppies"); };

HRESULT UseBstr(BSTR bst);
void call_bstr_function() {
    UseBstr(g_bst.get());

    // Unnecessary local allocation
    auto propName = wil::make_bstr_failfast(L"waffleCount");
    UseBstr(propName.get());

    // !!super mega ultra danger!! - not passing a real bstr...
    UseBstr(L"kablooie");
}

Instead, literals:

const auto g_bst = L"kittens"_bstr;
const auto g_bst2 = L"puppies"_bstr;

void call_bstr_function() {
    UseBstr(g_bst);
    UseBstr(L"waffleCount"_bstr);
}

(Thanks Copilot!)