Avoid blocks for static allocas and loading the closure environment

These blocks were required because previously we could only insert
instructions at the end of blocks, but we wanted to have all allocas in
one place, so they can be collapse. But now we have "direct" access the
the LLVM IR builder and can position it freely. This allows us to use
the same trick that clang uses, which means that we insert a dummy
"marker" instruction to identify the spot at which we want to insert
allocas. We can then later position the IR builder at that spot and
insert the alloca instruction, without any dedicated block.

The block for loading the closure environment can now also go away,
because the function context now provides the toplevel block, and the
translation of the loading happens first, so that's good enough.

Makes the LLVM IR a bit more readable, saving a bunch of branches in the
unoptimized code, which benefits unoptimized builds.