Also add a corresponding check for capabilities in the validator.
Update previously existing test cases where an instruction used to fail
assembling because of a version check, but now they succeed because the
instruction is also guarded by a capability. Now it should assemble.
Add tests to ensure that capabilities are checked appropriately.
The explicitly reserved instructions OpImageSparseSampleProj*
now assemble, but they fail validation.
Fixes#1624
According to the SPIRV Spec (2.16.1):
* There is at least one OpEntryPoint instruction, unless the Linkage
capability is being used.
* No function can be targeted by both an OpEntryPoint instruction and an
OpFunctionCall instruction.
Also updated unit tests to includ OpEntryPoint.
