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 part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

The new printf writer design focuses on optimizing the fast path. It
inlines any write to a buffer or string, and by handling buffering
itself can more effectively work with both internal and external file
implementations. The overflow hook should allow for expansion to
asprintf with minimal extra code.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D153999

The FormatSection and the writer functions both previously took a char*
and a length to represent a string. Now they use the StringView class to
represent that more succinctly. This change also required fixing
everywhere these were used, so it touches a lot of files.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D131994

After adding sprintf, snprintf is simple. The functions are very
similar. The tests only cover the behavior of the max length since the
sprintf tests should cover the other behavior.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D125826

This adds the sprintf entrypoint, as well as unit tests. Currently
sprintf only supports %%, %s, and %c, but the other conversions are on
the way.

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D125573