Unify symbol reference handling

[xlauko]

• makes all references use FlatSymbolRefs
• removes SSA from variable resolution
• introduces Cell and InitCell for cleaner Var -> Alloca lowering

[github-actions]

Cpp-Linter Report ⚠️

Some files did not pass the configured checks!

clang-format reports: 21 file(s) not formatted

• include/vast/CodeGen/ScopeContext.hpp
• include/vast/Conversion/Common/Mixins.hpp
• include/vast/Conversion/Passes.hpp
• include/vast/Conversion/TypeConverters/HLToStd.hpp
• include/vast/Conversion/TypeConverters/TypeConverter.hpp
• include/vast/Conversion/TypeConverters/TypeConvertingPattern.hpp
• include/vast/Dialect/Core/CoreTraits.hpp
• include/vast/Dialect/HighLevel/HighLevelOps.hpp
• include/vast/Util/Scopes.hpp
• lib/vast/CodeGen/CodeGenFunction.cpp
• lib/vast/CodeGen/DefaultDeclVisitor.cpp
• lib/vast/Conversion/FromHL/Passes.cpp
• lib/vast/Conversion/Generic/LowerValueCategories.cpp
• lib/vast/Conversion/ToMem/RefsToSSA.cpp
• lib/vast/Conversion/ToMem/StripParamLValues.cpp
• lib/vast/Conversion/ToMem/VarsToCells.cpp
• lib/vast/Dialect/HighLevel/HighLevelOps.cpp
• lib/vast/Dialect/HighLevel/HighLevelTypes.cpp
• lib/vast/Dialect/HighLevel/HighLevelVar.cpp
• lib/vast/Dialect/HighLevel/Transforms/LowerElaboratedTypes.cpp
• lib/vast/Dialect/HighLevel/Transforms/SpliceTrailingScopes.cpp

clang-tidy reports: 1 concern(s)

• include/vast/Dialect/Core/Interfaces/SymbolInterface.hpp:25:10: error: [clang-diagnostic-error]

  'vast/Dialect/Core/Interfaces/SymbolInterface.h.inc' file not found

     25 | #include "vast/Dialect/Core/Interfaces/SymbolInterface.h.inc"
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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