[clang] HLSL availability diagnostics design doc (PR #92207)

[Helena Kotas via cfe-commits]

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+=============================
+HLSL Availability Diagnostics
+=============================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+HLSL availability diagnostics emits errors or warning when unavailable shader APIs are used. Unavailable shader APIs are APIs that are exposed in HLSL code but are not available in the target shader stage or shader model version.
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hekota wrote:

Done.

https://github.com/llvm/llvm-project/pull/92207