Implement the THIR unsafety checker

[LeSeulArtichaut]

Unsafe operations

• CallToUnsafeFunction (Introduce the beginning of a THIR unsafety checker rust#83129)
• UseOfInlineAssembly (Check for inline assembly in THIR unsafeck rust#85259)
• InitializingTypeWith (Check for InitializingTypeWith and CastOfPointerToInt in THIR unsafeck rust#85381)
• CastOfPointerToInt (Check for InitializingTypeWith and CastOfPointerToInt in THIR unsafeck rust#85381)
• BorrowOfPackedField (removed -- see make unaligned_references future-incompat lint warn-by-default rust#82525)
• UseOfMutableStatic (Check for use of mutable/extern statics in THIR unsafeck rust#85419)
• UseOfExternStatic (Check for use of mutable/extern statics in THIR unsafeck rust#85419)
• DerefOfRawPointer (Add DerefOfRawPointer and CallToFunctionWith to THIR unsafeck rust#85306)
• AssignToDroppingUnionField (Implement AssignToDroppingUnionField in THIR unsafeck rust#87403)
• AccessToUnionField (Check for union field accesses in THIR unsafeck rust#85263)
• MutationOfLayoutConstrainedField (Implement Mutation- and BorrowOfLayoutConstrainedField in thir-unsafeck rust#86665)
• BorrowOfLayoutConstrainedField (Implement Mutation- and BorrowOfLayoutConstrainedField in thir-unsafeck rust#86665)
• CallToFunctionWith (Add DerefOfRawPointer and CallToFunctionWith to THIR unsafeck rust#85306)

[LeSeulArtichaut]

@Smittyvb thanks for your help on this! If you plan on working on more unsafe operations, it would be great if you posted a comment here saying that you're doing it so we don't risk duplicate work. Or, if you intend to work regularly on this, you can ask to join the project group so you can edit the comment above directly.