I got fed up with looking at error messages that tell me "VERIFICATION
FAILED: !is_error()". So this commit introduces DiagnosticEngine class
whose purpose is to accumulate and print more user-friendly errors.
For some reason I was afraid to add trivial accessors to classes
in earlier PRs, so we now have dozens of classes with public fields. I'm
not exactly looking forward to refactoring them all at once but I'll
do so gradually.
This commit introduces NamedVariableDeclaration and
SSAVariableDeclaration and allows storing both of them in Variable node.
Also, it adds additional structures in FunctionDefinition and
BasicBlock, which will be used to store SSA form related information.
This class stores a non-owning raw pointer to a member of `Node`, so
extra care is needed to ensure that referenced `Node`s will be alive
by the time `NodeSubtreePointer` is used. Since we only need to use this
class while traversing AST in `RecursiveASTVisitor`, access to class
methods can be restricted using `Badge<RecursiveASTVisitor>`.
Currently, they are not extremely useful, but the plan is to store
all function-local state in JSSpecCompiler::Function and all
"translation unit" state in ExecutionContext.
