Support MSVC 2013 (#12486)

* Added CV_CONSTEXPR macro

* Utilize CV_NOEXCEPT and CV_CONSTEXPR

* Provides some Ptr<> logical operators

* 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

* Fixed compilation error WITH_OPENMP under Windows

* Move up OpenMP condition

to share with opencv_contrib/xfeatures2d

* may be an typo fix

* remove identical branch,may be paste error

* add parentheses around macro parameter

* simplify if condition

* check malloc fail

* change the condition of branch removed by commit 3041502861318c85d901e8c08226ff67898c77ed

* simplify condition

* dims must > 0 or latter sz[dims-1] will underflow

* rewrote Mat::convertTo() and convertScaleAbs() to wide universal intrinsics; added always-available and SIMD-optimized FP16<=>FP32 conversion

* fixed compile warnings

* fix some more compile errors

* slightly relaxed accuracy threshold for int->float conversion (since we now do it using single-precision arithmetics, not double-precision)

* fixed compile errors on iOS, Android and in the baseline C++ version (intrin_cpp.hpp)

* trying to fix ARM-neon builds

* trying to fix ARM-neon builds

* trying to fix ARM-neon builds

* trying to fix ARM-neon builds

* trying to fix the custom AVX2 builder test failures (false alarms)

* fixed compile error with CPU_BASELINE=AVX2 on x86; raised tolerance thresholds in a couple of tests

* fixed compile error with CPU_BASELINE=AVX2 on x86; raised tolerance thresholds in a couple of tests

* fixed compile error with CPU_BASELINE=AVX2 on x86; raised tolerance thresholds in a couple of tests

* seemingly disabled false alarm warning in surf.cpp; increased tolerance thresholds in the tests for SolvePnP and in DNN/ENet