[HLSL] Make HLSL 202x the default mode in Clang

[llvm-beanz]

When we started working on HLSL in Clang we intended that HLSL 2021 would be Clang's target initial version. Things have changed since then, and we're now targeting HLSL 202x as the initial default version. This allows us to de-prioritize some legacy HLSL features that we'd prefer not to bring to Clang.

To support this the following changes should be made:

• Fix the HLSL 202x mode (there are some tests which currently fail or behave differently under it)
• Define HLSL 202y mode - [HLSL] Add HLSL 202y language mode #108437
• Re-define older versions in LangStandards.def to match 202x, and emit a warning when using them

[llvm-beanz]

PRs fixing broken test cases:

#108035
#108097
#108445