resolves #21038

* remove C assert

* revert C header

* fix several points in review

* fix test_ds.cpp

* Support cl_image conversion for CL_HALF_FLOAT (float16)

* Support cl_image conversion for additional channel orders:
  CL_A, CL_INTENSITY, CL_LUMINANCE, CL_RG, CL_RA

* Comment on why cl_image conversion is unsupported for CL_RGB

* Predict optimal vector width for float16

* ocl::kernelToStr: support float16

* ocl::Device::halfFPConfig: drop artificial requirement for OpenCL
  version >= 1.2. Even OpenCL 1.0 supports the underlying config
  property, CL_DEVICE_HALF_FP_CONFIG.

* dumpOpenCLInformation: provide info on OpenCL half-float support
  and preferred half-float vector width

* randu: support default range [-1.0, 1.0] for float16

* TestBase::warmup: support float16

(4.x) build: Xcode 12 support

* build: xcode 12 support, cmake fixes

* ts: eliminate clang 11 warnigns

* 3rdparty: clang 11 warnings

* features2d: eliminate build warnings

* test: warnings

* gapi: warnings from 18928

- apply tag 'skip_other'

properly handle "check" mode with:
"--perf_min_samples=1 --perf_force_samples=1"

- added functionality to collect memory usage of OpenCL sybsystem
- memory usage of fastMalloc() (disabled by default):
  * It is not accurate sometimes - external memory profiler is required.
- specify common `CV_TEST_TAG_` macros
- added applyTestTag() function
- write memory usage / enabled tags into Google Tests output file (.xml)

* core: drop usage of CV_USRTYPE1 in OpenCV

avoid OpenCV crashes due size change CV_ELEM_SIZE(CV_USRTYPE1): 8 -> 2

* ! fix persistence internal types

* added basic support for CV_16F (the new datatype etc.). CV_USRTYPE1 is now equal to CV_16F, which may break some [rarely used] functionality. We'll see

* fixed just introduced bug in norm; reverted errorneous changes in Torch importer (need to find a better solution)

* addressed some issues found during the PR review

* restored the patch to fix some perf test failures

during pre-commit testing

https://sourceforge.net/p/predef/wiki/OperatingSystems

fix compiler error when building with gnu-5 toolchain (eg as currently shipped with crystax.org android ndk, and presumably soon with android studio).

"ts_perf.cpp ....error: 'errno' was not declared in this scope
      int err=errno;"

--perf_instrument parameter now has int type and 0, 1, 2 modes (1 - simple trees, 2 - expanded trees for functions with same name but different calling address);

Maximum depth limit var was added to the instrumentation structure;

Trace names output console output fix: improper tree formatting could happen;
Output in case of error was added;

Custom regions improvements;

Improved timing and weight calculation for parallel regions; New TC (threads counter) value to indicate how many different threads accessed particular node;

parallel_for, warnings fixes and ReturnAddress code from Alexander Alekhin;

- calculate ticksTotal instead of ticksMean
- local / global width is based on ticksTotal value
- added instrumentation for OpenCL program compilation
- added instrumentation for OpenCL kernel execution

`has()` only tests for an argument presence which is always true for
arguments with default values. Use `get<bool>()` to check the value
instead.

Also:
- Silence clang warnings about unsupported command line arguments
- Add diagnostic print to calib3d test
- Fixed perf test relative error check
- Fix iOS build problem

IPP can be switched on and off on runtime;

Optional implementation collector was added (switched off by default in CMake). Gathers data of implementation used in functions and report this info through performance TS;

TS modifications for implementations control;