This PR reworks HLSL's implicit conversion sequences. Initially I was
seeking to match DXC's behavior more closely, but that was leading to a
pile of special case rules to tie-break ambiguous cases that should
really be left as ambiguous. We've decided that we're going to break
compatibility with DXC here, and we may port this new behavior over to
DXC instead.
This change is a bit closer to C++'s overload resolution rules, but it
does have a bit of nuance around how dimension adjustment conversions
are ranked. Conversion sequence ranks for HLSL are:
* Exact match
* Scalar Widening (i.e. splat)
* Promotion
* Scalar Widening with Promotion
* Conversion
* Scalar Widening with Conversion
* Dimension Reduction (i.e. truncation)
* Dimension Reduction with Promotion
* Dimension Reduction with Conversion
In this implementation I've folded the disambiguation into the
conversion sequence ranks which does add some complexity as compared to
C++, however this avoids needing to add special casing in
`CompareStandardConversionSequences`. I believe the added conversion
rank values provide a simpler approach, but feedback is appreciated.
The HLSL language spec updates are in the PR here:
https://github.com/microsoft/hlsl-specs/pull/261
