SPIRV-Tools support for SPIR-V 1.5 (#2865)

* Ensure same enum values have consistent extension lists

* val: fix checking of capabilities

The operand for an OpCapability should only be
checked for the extension or core version.
The InstructionPass registers a capability, and all its implied
sub-capabilities before actually checking the operand to an
OpCapability.

* Add basic support for SPIR-V 1.5

- Adds SPV_ENV_UNIVERSAL_1_5
- Command line tools default to spv1.5 environment
- SPIR-V 1.5 incorporates several extensions.  Now the disassembler
  prefers outputing the non-EXT or non-KHR names.  This requires
  updates to many tests, to make strings match again.
- Command line tests: Expect SPIR-V 1.5 by default

* Test validation of SPIR-V 1.5 incorporated extensions

Starting with 1.5, incorporated features no longer require
the associated OpExtension instruction.