Parser: block literals and __block variables

[dotcarmen]

blocked on/ part 2 of #969, wip towards closing #825

This PR implements parsing of block literals according to the language extension specification by Clang. Caveat: while the spec document specifies that the parameter list may only be omitted when the return type is omitted, Clang actually compiles programs that omit the return type but not the parameter list (excuse the linker error). The behavior here matches that behavior

Additionally, this change makes no attempt to track __block variable references within block bodies, since that's not necessary for the parse to complete, and that extra information would not be used anywhere yet. in the future, when implementing lowering support for blocks, this information tracking can be added

[dotcarmen]

i just realized aro doesn't support lowering to aarch64-macos - since macos is currently the only platform that works for testing blocks, i think this is enough to mark Blocks support as "completed" with the caveat that blocks don't get lowered, and an additional ticket should be opened for lowering literals and calls.

[dotcarmen]

^ still working on addressing review comments

[Vexu]

I added this warning in 2750279. You'll probably want to inline typeNameExtra here to handle decl attributes in the type properly:

int (^a)(int) = ^int __attribute__((const)) (int a){
    return a;
};