[clang] [flang] [flang][driver] Add pre-processing type for Fortran pre-processed files (PR #104664)

[Andrzej Warzyński via cfe-commits]

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@@ -79,7 +79,14 @@ TYPE("c++-module-cpp-output",    PP_CXXModule, INVALID,         "iim",    phases
 TYPE("ada",                      Ada,          INVALID,         nullptr,  phases::Compile, phases::Backend, phases::Assemble, phases::Link)
 TYPE("assembler",                PP_Asm,       INVALID,         "s",      phases::Assemble, phases::Link)
 TYPE("assembler-with-cpp",       Asm,          PP_Asm,          "S",      phases::Preprocess, phases::Assemble, phases::Link)
-TYPE("f95",                      PP_Fortran,   INVALID,         "i",      phases::Compile, phases::Backend, phases::Assemble, phases::Link)
+
+// Note: The `phases::Preprocess` phase is added to ".i" (i.e.
+// Fortran pre-processed) files. The reason is that Fortran
+// pre-processed files need further pre-proecessing when they
+// include modules from non-standard paths. In particular, we
+// need to add the search paths for these modules when flang
+// needs to emits pre-processed files.
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banach-space wrote:

> (...) The reason is that Fortran
// pre-processed files

Could you clarify that this isn't really about "Fortran pre-processed files", but more about "Fortran files pre-processed by LLVM Flang"?

> need further pre-proecessing when they
// include modules from non-standard paths

This is a bit misleading. IIUC, no further pre-processing happens, but the pre-processor "phase" has to be re-run to make sure that e.g. the include flags (i.e. `-I <dir>`) are preserved. That's because these include paths will contain module files (could `-I <dir>` define any other dirs?) and, unlike C header files, these module files wouldn't be included in the pre-processed file.

Please also add a note that `PP_TYPE` is set to `PP_Fortran` so that the driver is fine with "pre-processing a pre-processed file".

Hope this makes sense and apologies if I come across as pedantic. I just find this quite tricky and want to make sure that we throughly document these quirks for our future selves :)

https://github.com/llvm/llvm-project/pull/104664