`<xloctime>`: `do_get_monthname` and `do_get_weekday` do not set `eofbit` in some cases, but `libcxx` tests expect them to do

(I'm filing this issue to record my analysis about a few libcxx test failures related to get_monthname and get_weekday. These functions are super old, rarely used, and it's unclear what the correct behavior is.)

Describe the bug

When they reach the end of input after a successful match, time_get::do_get_monthname and time_get::do_get_weekday do not set eofbit, but libcxx tests expect them to do.

Command-line test case

D:\test>type test-time-get.cpp

#include <cassert>
#include <ctime>
#include <iostream>
#include <locale>

using I = const char*;

class my_facet : public std::time_get<char, I>
{
public:
    explicit my_facet(std::size_t refs = 0)
        : time_get(refs) {}
};

int main() {
    const my_facet f(1);
    std::ios str(nullptr);
    std::ios_base::iostate err;
    std::tm t;

    {
        std::cout << "Test 1: Get month name\n";

        const char in[] = "Jan";
        err = std::ios_base::goodbit;
        t = std::tm();
#ifdef TEST_GET_MONTHNAME_WEEKDAY
        I i = f.get_monthname(I(in), I(in+sizeof(in)-1), str, err, &t);
#else
        I i = f.get(I(in), I(in+sizeof(in)-1), str, err, &t, 'b');
#endif
        assert(i == in+3);
        assert(t.tm_mon == 0);
        if (err == std::ios::eofbit) {
            std::cout << "eofbit is set\n";
        } else {
            assert(err == std::ios::goodbit);
        }
    }

    {
        std::cout << "Test 2: Get weekday\n";
        const char in[] = "Sun";
        err = std::ios_base::goodbit;
        t = std::tm();
#ifdef TEST_GET_MONTHNAME_WEEKDAY
        I i = f.get_weekday(I(in), I(in+sizeof(in)-1), str, err, &t);
#else
        I i = f.get(I(in), I(in+sizeof(in)-1), str, err, &t, 'a');
#endif
        assert(i == in+3);
        assert(t.tm_mday == 0);
        if (err == std::ios::eofbit) {
            std::cout << "eofbit is set\n";
        } else {
            assert(err == std::ios::goodbit);
        }
    }
}

D:\test>cl /EHsc /DTEST_GET_MONTHNAME_WEEKDAY test-time-get.cpp

Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35725 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test-time-get.cpp
Microsoft (R) Incremental Linker Version 14.50.35725.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test-time-get.exe
test-time-get.obj

D:\test>cl /EHsc test-time-get.cpp /Fe: test-time-get-with-get.exe

Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35725 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test-time-get.cpp
Microsoft (R) Incremental Linker Version 14.50.35725.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test-time-get-with-get.exe
test-time-get.obj

D:\test>test-time-get.exe

Test 1: Get month name
Test 2: Get weekday

D:\test>test-time-get-with-get.exe

Test 1: Get month name
eofbit is set
Test 2: Get weekday
eofbit is set

Expected behavior

test-time-get.exe has the same output as test-time-get-with-get.exe.

STL version

https://github.com/microsoft/STL/commit/117ca965fe2f7157540edd9df5a62b0e42d7ece2

Additional context

Start with the eight listed libcxx time_get tests and the provided test-time-get.cpp reproduction, comparing the named get_monthname/get_weekday calls with the get overload. Check the standard requirements and existing time_get behavior before deciding whether eofbit should be set; done when the intended behavior is resolved and the affected tests agree.