spirv-val: Add --allow-vulkan-linkage option

[s-perron]

Adds a new option to spirv-val that permits the Linkage capability
when targeting Vulkan environments. This is needed to validate SPIR-V
generated by compilers that may still go through a linking process
before being provided to the driver.

• Adds allow_vulkan_linkage to spv_validator_options_t.
• Updates C and C++ validator option APIs to support setting this flag.
• Modifies CapabilityPass to allow Capability Linkage when targeting
  Vulkan if the option is enabled.
• Exposes the new --allow-vulkan-linkage flag in the spirv-val CLI.
• Adds corresponding tests for the new option and capability validation.

[alan-baker]

I haven't looked at the code, but a question: does it make sense to just use universal validation until linking has occurred? Do you expect imported resources, etc?

[spencer-lunarg]

Would it be simpler to just always always Linkage and have Validation Layers make sure at runtime you aren't trying to submit SPIR-V with a linker?

(only asking because I doubt the value of checking "ahead with spirv-val" for OpCapability Linkage is a real issue and would prevent yet-another-spirv-val-argument-flag)