Incorrect validation of DebugTypePointer: non-trivial base types are not allowed

When validating `DebugTypePointer` instructions, the base type is checked to be a trivial type defined by `DebugTypeBasic`. This means that pointers to vectors or matrices are considered invalid, which prevents variables such as `vec4 fragColor` (GLSL) to have debug info emitted.

https://github.com/KhronosGroup/SPIRV-Tools/blob/933db564ca660477b360480b8a1d7589d7c6694e/source/val/validate_extensions.cpp#L3170-L3177

This validation rule is not present in the `NonSemantic.Shader.DebugInfo.100` specification. Potential solutions:
(1) Check that `Base Type` in `DebugTypePointer` could be any debug type instruction.
(2) Simply remove the validation - `ValidateOperandBaseType(_, inst, 5, ext_inst_name)`
(3) Update the specification to reflect the intended behaviour in case pointers to non-trivial types are not allowed

Thanks for reading and checking that!