docgen relies on the convention that we have a file foo.cpp in
libc/src/\<header\>/. Because the above functions weren't in libc/src/strings/
but rather libc/src/string/, docgen could not find that we had implemented
these.

Rather than add special carve outs to docgen, let's fix up our sources for
these 7 functions to stick with the existing conventions the rest of the
codebase follows.

Link: #118860
Fixes: #118875

The build is currently broken and we don't have the resources to keep it
up to date :-/

Fix a few issues found when trying to build the benchmark:

Errors

1. Unable to find include "src/__support/macros/config.h" in
LibcMemoryBenchmarkMain.cpp

Warnings

2. Unused variable warning `Index` in MemorySizeDistributions.cpp
3. Fix deprecation warning for const-ref version of `DoNotOptimize`.
warning: 'DoNotOptimize<void *>' is deprecated: The const-ref version of
this method can permit undesired compiler optimizations in benchmarks

As specified in the docs,
1) raw_string_ostream is always unbuffered and
2) the underlying buffer may be used directly

( 65b13610a5226b84889b923bae884ba395ad084d for further reference )

Avoid unneeded calls to raw_string_ostream::str(), to avoid excess indirection.

This PR adds benchmarking for `atan2()`, `__nv_atan2()`, and
`__ocml_atan2_f64()` using the same setup as `sin()`. This PR also adds
support for throughout bencmarking for functions with 2 inputs.

Reverts llvm/llvm-project#102825

This is invalid in C++, and clang recently started warning on it as of
#101853

This PR adds benchmarking for `sinf()` using the same set up as `sin()`
but with a smaller range for floats.

Previously, `AmdgpuSinTwoPow_128` and others were too large for their
table cells. This PR shortens the name to `AmdSin...`

There were also some `-` missing in the separator. This PR instead
creates the separator string using the length of the headers.