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f8f0f932 - [HLSL][Matrix] Add APValue and ConstExpr evaluator support for matrices (#178762)

[HLSL][Matrix] Add APValue and ConstExpr evaluator support for matrices (#178762) Fixes #168935 This PR adds basic support for matrix APValues and a ConstExpr evaluator for matrices. - ConstExpr evaluation changes: - Matrix initializer list - Matrix HLSL elementwise cast - Matrix HLSL aggregate splat - Vector HLSL matrix truncation - Int HLSL matrix truncation - Float HLSL matrix truncation - Matrix APValue: - AST dumper and serialization - Value flattening Note that APValue matrices hold its elements in row-major order irrespective of the `-fmatrix-memory-layout` flag. The `-fmatrix-memory-layout` is for codegen, not semantics, so the decision of which memory layout to use for the matrix APValue can be independent of the memory layout for codegen. There are also a number of places expecting switch case coverage over all APValues but which do not currently support matrix APValues. I have added placeholder llvm_unreachables to these places for the matrix APValue, as these places can not currently be exercised in clang tests (AFAIK). Assisted-by: claude-opus-4.5