modules: #pragma once must appear before #ifdef when using -fmodules

[andrewjcg]

Bugzilla Link	39184
Version	trunk
OS	All
CC	@DougGregor

Extended Description

When compiling with -fmodules (but not necessarily actually building modules), #pragma once guards don't work when appearing after a #ifdef ... guard. Example failing test: https://reviews.llvm.org/differential/diff/168367/.

I couldn't find any supporting documentation saying that #pragma once must come first, but assuming this is true, then this probably isn't a bug (but maybe should then be a warning?).

[davidstone]

Given #38554, #pragma once doesn't work before the #ifdef guard, either, so this might just be a duplicate.