Functions like isalpha / tolower can operate on chars internally. This
allows us to get rid of unnecessary casts and open a way to creating
wchar_t overloads with the same names (e.g. for isalpha), that would
simplify templated code for conversion functions (see
315dfe5865962d8a3d60e21d1fffce5214fe54ef).

Add the int->char converstion to public entrypoints implementation
instead. We also need to introduce bounds check on the input argument
values - these functions' behavior is unspecified if the argument is
neither EOF nor fits in "unsigned char" range, but the tests we've had
verified that they always return false for small negative values. To
preserve this behavior, cover it explicitly.

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.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

This replaces the old macros LIBC_COPT_TEST_USE_FUCHSIA and
LIBC_COPT_TEST_USE_PIGWEED with LIBC_COPT_TEST_ZXTEST and
LIBC_COPT_TEST_GTEST, respectively.  These are really not about
whether the code is in the Fuchsia build or in the Pigweed build,
but just about what test framework is being used.  The gtest
framework can be used in many contexts, and the zxtest framework
is not always what's used in the Fuchsia build.

The test/UnitTest/Test.h wrapper header now provides the macro
LIBC_TEST_HAS_MATCHERS() for use in `#if` conditionals on use of
gmock-style matchers, replacing `#if` conditionals that test the
framework selection macros directly.

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

This is a follow up to https://reviews.llvm.org/D152503

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D152533

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D148452