translate-c: fix applying struct alignment to first field

[xdBronch]

fixes #23861, fixes #22766

[xdBronch]

failures feel obvious in hindsight but i believe stuff was getting cached so i didnt get any failures locally. this will need a check against the fields natural alignment which i wasnt able to figure out before, would appreciate some guidance there.

[mlugg]

Looks like you want clang::ASTContext::getTypeAlign: https://clang.llvm.org/doxygen/classclang_1_1ASTContext.html#afb825a845dced571b094627bfcae7870
We actually seem to have an unused wrapper for this:

zig/src/zig_clang.cpp

Lines 2463 to 2465 in bc3c50c

	unsigned ZigClangASTContext_getTypeAlign(const ZigClangASTContext* self, ZigClangQualType T) {
	return reinterpret_cast<const clang::ASTContext *>(self)->getTypeAlign(bitcast(T));
	}

You'll just want to add a corresponding declaration on the Zig side here:

zig/src/clang.zig

Lines 164 to 167 in bc3c50c

	pub const ASTContext = opaque {
	pub const getPointerType = ZigClangASTContext_getPointerType;
	extern fn ZigClangASTContext_getPointerType(*const ASTContext, T: QualType) QualType;
	};

And use that to get the field type's natural alignment.

[xdBronch]

apologies for not finishing this, the logic ended being somewhat tedious and i ended up forgetting about it. im going to just close this since its already properly done in https://github.com/ziglang/translate-c + #24432 (comment)