[SPIR-V] Add SPIRV-Tools for testing (#73044)

Add spirv-dis (disassembler) and spirv-val (validator) from SPIRV-Tools
as external dependencies for testing the SPIR-V backend. These tools are
test dependencies only.

SPIR-V backend tests now have a dependency on the spirv-dis and
spirv-val targets when the `LLVM_INCLUDE_SPIRV_TOOLS_TESTS` cmake
variable is set, which allows additional test files with the `REQUIRES:
spirv-tools` constraint to run, along with additional `RUN: %if
spirv-tools ...` lines in existing tests. All other SPIR-V backend tests
will run normally when `LLVM_INCLUDE_SPIRV_TOOLS_TESTS` is not set.

Several tests are included to show these tools' use, however more tests
will be migrated and added later.
* OpVariable_order.ll shows how spirv-val can catch bugs in the backend.
* basic_int_types_spirvdis.ll shows how tests can be much shorter and
more readable by FileChecking the spirv-dis output.
* basic_int_types.ll shows how an additional RUN line can add validation
to ex...