Signature packing, codegen fixes, and validation fixes and additions.

[tex3d]

• Add DxilSignatureAllocator for signature packing
• Fix signature validation. Add more validation.
• Fix and add validation tests.
• Fix codegen for inout params with SV like SV_Coverage
• fix m_SemanticStartIndex on DxilSignatureElement::Initialize
• fix DxilSignatureElement::GetColsAsMask for start col == 2
• Add diags for signature allocation failures
• Use Regex in ValidationTest

[msftclas]

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