[0029] Add separate enums for the different types of interpretation

[damyanp]

NOTE: this currently targets a copy of @anupamachandra's PR branch for microsoft#407.
Once that PR is completed, this one will be retargetted to hlsl-spec's main.

It looks like we'll need to start the review process from scratch once we retarget the microsoft/hlsl-specs repo.

Previously, the spec used a single enum to represent types in various contexts:

• to describe how to interpret input/output vectors
• to describe how to interpret matrices/bias vectors loaded from memory
• to describe the element type of HLSL vectors in calls to CheckFeatureSupport

Each of these has a separate set of valid values. This change uses different enums to express these different possible values.

Closes microsoft#402

[damyanp]

Now tracked by this PR: microsoft#409