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.

Reverts llvm/llvm-project#143810 

This PR breaks our buildbot:
https://lab.llvm.org/buildbot/#/builders/10/builds/7159 revert to
unblock downstream merge.

In #143802 the stdio test cleanup missed a few places where errno was
being set to a failing value, and one where the framework needed to
included.

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

https://github.com/llvm/llvm-project/issues/60481

Summary:
The GPU handling for a lot of `FILE *` functions pretty much just
forwards it to the host via RPC. This test checks for implementation
defined behavior, which sometimes passes and sometimes doesn't. We just
disable it here so it works on the standard semantics.

We do this forwarding primarily for interopt w/ the host if the user is
compiling from an offloading language (e.g. CUDA).