…ecting the

alignment of `byval` on x86 in the process.

Commit 88e4d2c from five years ago removed
support for alignment on indirectly-passed arguments because of problems with
the `i686-pc-windows-msvc` target. Unfortunately, the `memcpy` optimizations I
recently added to LLVM 16 depend on this to forward `memcpy`s. This commit
attempts to fix the problems with `byval` parameters on that target and now
correctly adds the `align` attribute.

The problem is summarized in [this comment] by @eddyb. Briefly, 32-bit x86 has
special alignment rules for `byval` parameters: for the most part, their
alignment is forced to 4. This is not well-documented anywhere but in the Clang
source. I looked at the logic in Clang `TargetInfo.cpp` and tried to replicate
it here. The relevant methods in that file are
`X86_32ABIInfo::getIndirectResult()` and
`X86_32ABIInfo::getTypeStackAlignInBytes()`. The `align` parameter attribute
for `byval` parameters in LLVM must match the platform ABI, or miscompilations
will occur. Note that this doesn't use the approach suggested by eddyb, because
I felt it was overkill to store the alignment in `on_stack` when special
handling is really only needed for 32-bit x86.

As a side effect, this should fix rust-lang#80127, because it will make the `align`
parameter attribute for `byval` parameters match the platform ABI on LLVM
x86-64.

[this comment]: rust-lang#80822 (comment)

From the test logs, other extern-fn-* tests have this:

[run-make] tests/run-make/extern-fn-with-packed-struct ... ignored, ignored when cross-compiling
[run-make] tests/run-make/extern-fn-with-union ... ignored, ignored when cross-compiling
[run-make] tests/run-make/extern-multiple-copies ... ignored, ignored when cross-compiling
[run-make] tests/run-make/extern-multiple-copies2 ... ignored, ignored when cross-compiling
[run-make] tests/run-make/extern-overrides-distribution ... ignored, ignored when cross-compiling
[run-make] tests/run-make/extra-filename-with-temp-outputs ... ignored, ignored when cross-compiling
[run-make] tests/run-make/extern-fn-explicit-align ... FAILED

x86 Windows also should not use byval since the struct is
overaligned, see https://reviews.llvm.org/D72114

…d alignment

…ct with natural alignment > 8 by reference

…nd repr(transparent)

…ent, improve assertions

...which seems not to be available on some platforms.
Or maybe it is under a different name but I don't want to deal with that

Instead, use two u64s. This isn't exactly the same, but we already have
some coverage of the packed u128 case in another test, so it's not
essential to have it here.

…ively