[Decompiler] Begin ir2 form implementation

[water111]

Some random thoughts:

• Look at how to pack in the old IR_Atomic info. (std::optional<AtomicInfo>?)
• Look at how to map atomic op idx to FormElements? Or how will we do variable scoping?
• How to get all variables that are used?
• Regression tests (offline)
• Make sure it runs even if we don't have type info

[coveralls]

Pull Request Test Coverage Report for Build 492314448

• 5 of 1216 (0.41%) changed or added relevant lines in 10 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-1.6%) to 55.436%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
decompiler/IR2/AtomicOpTypeAnalysis.cpp	0	3	0.0%
decompiler/IR2/AtomicOp.h	0	14	0.0%
decompiler/ObjectFile/ObjectFileDB_IR2.cpp	0	22	0.0%
decompiler/Function/CfgVtx.cpp	0	26	0.0%
decompiler/IR2/AtomicOpForm.cpp	0	35	0.0%
decompiler/IR2/Form.h	0	70	0.0%
decompiler/IR2/Form.cpp	0	373	0.0%
decompiler/IR2/cfg_builder.cpp	0	668	0.0%

Files with Coverage Reduction	New Missed Lines	%
decompiler/Function/CfgVtx.h	1	0%
decompiler/IR2/AtomicOpTypeAnalysis.cpp	1	0%
decompiler/IR2/AtomicOp.h	2	55.56%

Totals
Change from base Build 490254738:	-1.6%
Covered Lines:	21766
Relevant Lines:	39263

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💛 - Coveralls