sema: clean-up {union,struct}FieldAlignment and friends

My main gripes with this design were that it was incorrectly namespaced, the naming was inconsistent and a bit wrong (`fooAlign` vs `fooAlignment`).

This commit moves all the logic from `PerThread.zig` to use the zcu + tid system that the previous couple commits introduce.
I've organized and merged the functions to be a bit more specific to their own purpose.

- `fieldAlignment` takes a struct or union type, an index, and a Zcu (or the Sema version which takes a Pt), and gives you the alignment of the field at the index.
- `structFieldAlignment` takes the field type itself, and provides the logic to handle special cases, such as externs.

A design goal I had in mind was to avoid using the word 'struct' in the function name, when it worked for things that aren't structs, such as unions.

src/mutable_value.zig

@@ -223,7 +223,7 @@ pub const MutableValue = union(enum) {
                                     @memset(elems[0..@intCast(len_no_sent)], .{ .interned = undef_elem });
                                 },
                                 .Struct => for (elems[0..@intCast(len_no_sent)], 0..) |*mut_elem, i| {
-                                    const field_ty = ty.structFieldType(i, zcu).toIntern();
+                                    const field_ty = ty.fieldType(i, zcu).toIntern();
                                     mut_elem.* = .{ .interned = try pt.intern(.{ .undef = field_ty }) };
                                 },
                                 else => unreachable,