Add missing attributes to indirect calls for foreign functions

When calling a foreign function, some arguments and/or return value
attributes are required to conform to the foreign ABI. Currently those
attributes are only added to the declaration of foreign functions. With
direct calls, this is no problem, because LLVM can see that those
attributes apply to the call. But with an indirect call, LLVM cannot do
that and the attribute is missing.

To fix that, we have to add those attribute to the calls to foreign
functions as well.

This also allows to remove the special handling of the SRet attribute,
which is ABI-dependent and will be set via the `attr` field of the
return type's `ArgType`.
Update LLVM

To fix #8106, we need an LLVM version that contains r211082 aka 0dee6756
which fixes a bug that blocks that issue.

There have been some tiny API changes in LLVM, and cmpxchg changed its
return type. The i1 part of the new return type is only interesting when
using the new weak cmpxchg, which we don't do.
Compile bools to i1

We currently compiled bools to i8 values, because there was a bug in
LLVM that sometimes caused miscompilations when using i1 in, for
example, structs.

Using i8 means a lot of unnecessary zero-extend and truncate operations
though, since we have to convert the value from and to i1 when using for
example icmp or br instructions. Besides the unnecessary overhead caused
by this, it also sometimes made LLVM miss some optimizations.

Fixes #8106.