In a nutshell, when we understand that the expression is a function
call (this happens at '(' after an expression), stop parsing expression
and start parsing function arguments. This makes
`FunctionCallCanonicalizationPass` and the workaround for zero argument
function calls obsolete.
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.
It simplifies ladders of BinaryOperators nodes in the function call
arguments into nice and neat FunctionCall node. Ladders initially appear
since I do not want to complicate expression parser, so it interprets
`f(a, b, c, d)` as `f "function_call_operator" (a, (b, (c, d))))`.
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.
