If the TLS key is 0-sized, then the linux linker is apparently smart enough to
put everything at the same pointer. OSX on the other hand, will reserve some
space for all of them. To get around this, the TLS key now actuall consumes
space to ensure that it gets a unique pointer

Closes #3273

I think of this as a stylistic opinion which shouldn't necessarily be enforced
by default on all users of rust, but that's just my opinion.

We used to have concrete types in glue functions, but the way we used
to implement that broke inlining of those functions. To fix that, we
converted all glue to just take an i8* and always casted to that type.

The problem with the old implementation was that we made a wrong
assumption about the glue functions, taking it for granted that they
always take an i8*, because that's the function type expected by the
TyDesc fields. Therefore, we always ended up with some kind of cast.

But actually, we can initially have the glue with concrete types and
only cast the functions to the generic type once we actually emit the
TyDesc data.

That means that for glue calls that can be statically resolved, we don't
need any casts, unless the glue uses a simplified type. In that case we
cast the argument. And for glue calls that are resolved at runtime, we
cast the argument to i8*, because that's what the glue function in the
TyDesc expects.

Since most of out glue calls are static, this saves a lot of bitcasts.
The size of the unoptimized librustc.ll goes down by 240k lines.

If an "if" expression has no "else", we don't have to create an LLVM
basic block either.

When there are no allocas, we don't need a block for them.

Currently, we always create a dedicated "return" basic block, but when
there's only a single predecessor for that block, it can be merged with
that predecessor. We can achieve that merge by only creating the return
block on demand, avoiding its creation when its not required.

Reduces the pre-optimization size of librustc.ll created with --passes ""
by about 90k lines which equals about 4%.

This prevents attempting to run deallocated code (which is no longer present
always)

Also ends up fixing one case in libstd

Also reworks a bunch of type_use to make things work for default
static methods soon.

Currently, immediate values are copied into an alloca only to have an
addressable storage so that it can be used with memcpy. Obviously we
can skip the memcpy in this case.

Closes #6004

`malloc` already returns memory correctly aligned for every possible
type in standard C, and that's enough for all types in Rust too

be limited by the innermost repeating scope.

Fixes #7336.