This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

This breaks builds of libc with top of tree clang under -Werror.

This pr is to resovle #126948

---------

Co-authored-by: Joseph Huber <huberjn@outlook.com>

Summary:
Currently we dispatch the writing mode off of a runtime enum passed in
by the constructor. This causes very unfortunate codegen for the GPU
targets where we get worst-case codegen because of the unused function
pointer for `sprintf`. Instead, this patch moves all of this to a
template so it can be masked out. This results in no dynamic stack and
uses 60 VGPRs instead of 117. It also compiles about 5x as fast.

This is a (no-op) locale version of strftime.

Fixes: https://github.com/llvm/llvm-project/issues/106630

Reverts llvm/llvm-project#127708

This is a (no-op) locale version of strftime.

Forgot to change a size_t to an int, which caused warnings on gcc but
not clang for some reason. Regardless, this patch fixes the issue.

Implements the posix-specified strftime conversions for the default
locale, along with comprehensive unit tests. This reuses a lot of design
from printf, as well as the printf writer.

Roughly based on #111305, but with major rewrites.

On some systems time_t is 32 bit, causing build errors (with -Werror)
in get_epoch which attempts to implicitly convert an int64_t to a
time_t.

Fixes:

error: implicit conversion loses integer precision: 'int64_t' (aka 'long
long') to 'time_t' (aka 'int') [-Werror,-Wshorten-64-to-32]
      332 |     return mktime_internal(timeptr);
          |     ~~~~~~ ^~~~~~~~~~~~~~~~~~~~~~~~