This reverts commit 92a116c4ef822950f8c57eaa5164c844c73a1f7e with a fix
for fgets test - convert nullptr to fgets return type (char*), since the
matcher is pedantic.

Reverts llvm/llvm-project#143972 - matcher seems to be pedantic for
fgets tests, reverting to verify and fix.

This reverts commit a93e55e57ed00a55f822c64e3520c7c732b58480 and fixes
build and test failures:

* Proper include added to setvbuf_test.cpp
* fgetc/fgetc_unlocked/fgets tests are ported to ErrnoSetterMatcher and
are made more precise. This fixes inconsistencies between expectations
in regular and GPU builds - ErrnoSetterMatcher is configured to omit
errno matching on GPUs, as fgetc implementation on GPU doesn't set
errno, in contrast to Linux.

Reverts llvm/llvm-project#143802. Follow-up fix
3c7af175e51c3ab08ac3c442146c2b822f38c01e wasn't robust enough and itself
got reverted.

Reduce the direct use of libc_errno in stdio unit tests by adopting
ErrnoCheckingTest where appropriate.

Also removes the libc_errno.h inclusions from stdlib.h tests that were
accidentally added in d87eea35fac5a34a841c637db8908128409a184e

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

* Get rid of libc_errno assignments in str_to_* __support tests, since
those API have been migrated to return error in a struct instead.
* Migrate tests for atof and to strto* functions from <stdlib.h> and for
strdup from <string.h> to use ErrnoCheckingTest harness.

The a0c4f854cad2b97e44a1b58dc1fd982e1c4d60f3 change replaced the
local int_to_b36_char function returning `char` with uses of the
__support function of the same name that returns `int`.  The uses
of the old local function lacked the casts that all other uses of
the shared function of the same name had.  Add them.

The previous implementation of the ctype functions assumed ASCII.
This patch changes to a switch/case implementation that looks odd, but
actually is easier for the compiler to understand and optimize.

Having libc_errno outside of the namespace causes versioning issues when
trying to link the tests against LLVM-libc. Most of this patch is just
moving libc_errno inside the namespace in tests. This isn't necessary in
the function implementations since those are already inside the
namespace.