Avoid creating llenv blocks when there's nothing to load

Currently, all closures have an llenv block to load values from the
captured environment, but for closure that don't actually capture
anything, that block is useless and can be skipped.
Remove an unnecessary block/jump from the drop glue for @-pointer

The nested with_cond calls each introduce a "next" block, with the
inner one just jumping to the outer one.
Provide lower level access to the LLVM IR builder

Currently, the helper functions in the "build" module can only append
at the end of a block. For certain things we'll want to be able to
insert code at arbitrary locations inside a block though. Although can
we do that by directly calling the LLVM functions, that is rather ugly
and means that somethings need to be implemented twice. Once in terms
of the helper functions and once in terms of low level LLVM functions.

Instead of doing that, we should provide a Builder type that provides
low level access to the builder, and which can be used by both, the
helper functions in the "build" module, as well larger units of
abstractions that combine several LLVM instructions.
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.
auto merge of #7941 : dotdash/rust/codegen, r=huonw

These changes remove unnecessary basic blocks and the associated branches from
the LLVM IR that we emit. Together, they reduce the time for unoptimized builds
in stage2 by about 10% on my box.