Use the correct LLVM integer sizes

Use the integer sizes LLVM uses, rather than having random projections
laying around. Sizes are u64, Alignments are u32, C_*int is target-dependent
but 64-bit is fine (the int -> C_int conversion is non-precision-losing,
but it can be preceded by `as int` conversions which are, so it is
somewhat ugly. However, being able to suffix a `u` to properly infer
integer types is nice).
Add a compile-time error when oversized types are used

LLVM generates wrong code (which may be an instance of compile-time UB) when
faced with types that take lots of memory - bigger than the address space.
Make using such types a trans error. While trans errors are bad, overbig
types are expected to be very rare.
add some tests
response for review comments
prevent the creation of integers too big for the target architecture
improve the comments on integer overflow safety
fix stupid error
fix a failing test
fix test patterns - should rebase the commits properly
Make the tests green as they should on 32-bit architectures

On 32-bit architectures, the size calculations on two of the tests wrap-around
in typeck, which gives the relevant arrays a size of 0, which is (correctly)
successfully allocated.
auto merge of #18041 : arielb1/rust/no-size-overflow, r=pnkfelix

Should fix #17913.

Also clean-up u64/u32-ness. I really should split this commit and add tests (I have no idea how to add them).