Make pointers to isomorphic type interchangeable with option. (#2570)

* Make pointers to logically matching types interchangeable with option.

DXC will be generating code where the function parameters will be a more
generic type that the actual parameter.  They should be logically
matching and the decorations of the actual parameter must be a superset
of the decorations of the formal parameter.

We want to accept this code with an options so that spirv-opt can then
inline and fix the type mismatch.  We will accept this under a new
options `--before-hlsl-legalization`.

The new option will also imply `relax-logical-pointer` so that HLSL
frontends will need to use just the one more generic option.

Moved the |LogicallyMatches| to the validation state to make it
available in more places.  Also added a parameter to have it check the
decorations.  I did not do a separate function for the decorations
because checking the decorations involves making sure the types
logically match anyway.

Fixes #2535