[clang] [clang][modules] Headers meant to be included multiple times can be completely invisible in clang module builds (PR #83660)

[James Y Knight via cfe-commits]

jyknight wrote:

Oh -- I'd also note that even if this is reverted, ilya-biryukov may want to continue to investigate the source-location issue -- it's entirely possible that the correct fix will trigger that same problem!

https://github.com/llvm/llvm-project/pull/83660