Update SPIRV-Tools, SPIRV-Headers

Headers update fixes validation of spv.shaderBallotAMD.

SPV 1.4: Add code generation for SPIR-V 1.4 features

...when doing a variable lookup on an array of constants.

This includes the test for the first feature of OpEntryPoint.

Latest known-good SPIRV-Tools: WARNING: Needs python 3.x.

You may need to install python 3.x and also
[windows users] may need to get it in your path in front of python 2.7.x.

If you don't need the external projects (i.e., SPIRV-Tools) and don't want
to upgrade to python 3, don't create the External subdirectory and CMake
won't require python 3.  (CMake has a bug that makes it find the wrong one
on a first pass if the top level does not require the version needed by
lower levels.)

For nonuniformEXT constructor, make a copy of the node to decorate

Add support for GL_EXT_buffer_reference2

Fix issues of explicit conversions.

If a shader's only use of float16 is for FConvert, add the Float16 capability

When visiting instructions, check that there are no 16-bit storage capabilities
and add the Float16 capability. Same for int8/int16.

For certain int8/int16/float16 conversions,they are conditionally allowed
when extensions defining explicit arithmetic types are enabled.

This was added some time ago, but I suspect largely unused.
Christoph pointed out of few contradictions to actual convention,
so that's fixed. But, I suspect it is largely incomplete.

introduce ability to dump builtin symbol table

add corresponding EShMsgBuiltinSymbolTable
TSymbol::dump functions have option to do "complete" print
bugfix in TType::getCompleteString, structure can be null for block

Add cstdlib include

On some platforms some of the includers of this header don't have
'atoi' defined after the change in #1749