Add diagnostics, derivative uniformity failures trigger diagnostics

- Define diagnostics, each having:
    - severity: error, warning, info, off
    - triggering rule
    - triggering location
- The spec names certain triggering rules.
  Currently only "derivative_uniformity" is defined, and it
  defaults to "error" severity.
- Uniformity section is updated generally to say that analysis
  failures trigger a "uniformity failure", instead of "rejects the program".
  In one place, say that uniformity failure for deriviative builtins
  trigger derivative_uniformity at the call site of that builtin;
  and that uniformity failures for barrier builtins trigger a
  shader-creation error.
- Triggering rules are named like identifiers.
- Define diagnostic filters, which have
    - affected range: span of source text
    - new severity
    - triggering rule
- Filters with an unknown triggering rule are ignored.
- There are global filters, which are given as directives at
   the top of the module (mixed in with enables).
- There are "range" filters, which are given as @diagnostic
   attributes before: control flow statements, switch bodies,
   function declarations, and compound statements.
- @diagnostic attributes may be given more than once on a particular
  object.  But they must not "conflict", meaning have the same
  range and triggering rule, but different "new severity".
- The affected ranges of diagnostic filters perfectly nest, by
  constrution.
- To apply a filter to a diagnostic:
    - the triggering location must fall in the affected range
    - they must have the same triggering rule
    - the severity of the diagnostic is updated to the one specified
      in the filter.
- Define a step-wise process for processing triggered filters, roughly:
  1. For each diagnostic with triggering rule R, apply the diagnostic
     filter for rule R whose affected range is smallest but also
     encloses the diagnostic's triggering location, if one exists.
  2. Discard "off" diagnostics.
  3. For each rule R, all but one "info" diagnostic for R may be
     discarded.
  4. For each rule R, all but one "warning" diagnostic for R may be
     discarded. If there is such a diagnostic, all the info messages
     for R may be discarded.
  5. If there is at least one error diagnostic, all other diagnostics
     may be discarded.
     Generate either a shader-creation or pipeline-creation error.
  6. Diagnostics generated during shader-creation are reported
     in the messsages member of the GPUConpilationInfo object.
  7. Error diagnostics generated during pipeline-creation are reported
     to the enclosing GPU error scope.

Fixes: gpuweb#3554 gpuweb#3689

Author: dneto0