Reuse code for Metal and WGSL entry point legalization (shader-slang#6063)

* Refactor to reuse common for metal and wgsl entry point legalization

* refactor system val work item

* refactor simplify user names

* clean up fix semantic field of struct

* improve code layout

* split wgsl/metal to seperate classes and cleanup

* remove extra includes

* remove dead code comments

* minor cleanup

* squash merge from master and resolve conflict

* apply metal spec const thread count changes

* Revert "apply metal spec const thread count changes"

This reverts commit c42d707.

* Revert "squash merge from master and resolve conflict"

This reverts commit 06db88e.

* Merge remote-tracking branch 'origin/master'

* apply metal spec const thread count changes

* Revert "apply metal spec const thread count changes"

This reverts commit 3b9e6f5.

* Revert "Merge remote-tracking branch 'origin/master'"

This reverts commit 99869d5.

* apply metal spec const thread count changes

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Co-authored-by: Yong He <yonghe@outlook.com>

Author: fairywreath