Allow precision qualifiers in function params.

Previously, `Type::applyPrecisionQualifiers` would return a new type
(e.g. `mediump + float` returned `half`) but left the precision
qualifier flags as-is. This was implemented that way because the
modifiers were already baked into a pool, so mutating them was
difficult.

The rewritten DSLParser does not share this limitation--every place
where applyPrecisionQualifiers is used, the Modifiers are easily
mutable. As a result, `applyPrecisionQualifiers` can now clear the
precision-qualifier bits on the Modifier, meaning that `half` and a
`mediump float` will generate the exact same Type/Modifier combination.

This change fixes a bug where precision qualifiers were not allowed on
function parameters. (See `check_parameters` in FunctionDeclaration.cpp
to pinpoint the cause of the error. A less-invasive fix could have just
marked those modifier bits as allowed in `check_parameters`, but this
fix addresses the root of the issue and is honestly how I wanted
`applyPrecisionQualifiers` to work all along.)

Change-Id: I331813efa54138f469a0d5bff2d274cd3ce64b70
Bug: skia:12489
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/455156
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
9 files changed