`<xloctime>`: `time_get::do_get` doesn't parse a literal `%`

[cpplearner]

Describe the bug

MSVC STL's time_get::do_get doesn't know the %% specifier. It should recognize this specifier and consume a literal %.

Command-line test case

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

#include <cassert>
#include <ctime>
#include <iomanip>
#include <sstream>

int main() {
    std::tm t{};
    std::stringstream ss{"% "};
    ss >> std::get_time(&t, "%%");
    assert(!ss.fail());
}

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

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

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

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

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

Assertion failed: !ss.fail(), file test-time-get-do-get.cpp, line 10

Expected behavior

Assertion passed.

STL version

117ca96

Additional context

Both #6129 and this issue concern time_get::do_get (and affect the same libcxx test file), but they can be fixed independently.

In fact, IMO this issue is much easier to fix and could be a "good first issue".

[StephanTLavavej]

Locale bugs are too scary for me to mark as good first issue.