Use the Carbon-computed alignment for allocas, loads, stores, and
memcpys. Previously we used whatever LLVM felt like giving us, which
would result in ABI mismatches and runtime crashes due to misalignment
when creating objects of imported C++ class types, as well as resulting
in some surprising choices like `(i32, i32)` and `()` having 8-byte
alignment instead of 4 and 1, respectively.
Richard Smith
b7f11e4c61