[clang] [C++20][Modules] Implement P1857R3 Modules Dependency Discovery (PR #107168)

[Hubert Tong via cfe-commits]

================
@@ -986,6 +989,14 @@ def warn_module_conflict : Warning<
   InGroup<ModuleConflict>;
 
 // C++20 modules
+def err_pp_module_name_is_macro : Error<
+  "%select{module|partition}0 name component %1 cannot be a object-like macro">;
+def err_pp_module_expected_ident : Error<
+  "expected %select{identifier after '.' in |}0module name">;
+def err_pp_module_decl_in_header
+    : Error<"module declaration must not come from an #include directive">;
+def err_pp_cond_span_module_decl
+    : Error<"preprocessor conditionals shall not span a module declaration">;
----------------
hubert-reinterpretcast wrote:

These are both expressions of the fact that a `module` directive is neither a _control-line_ nor a _text-line_. Which is to say that there should be a preprocessor diagnostic (effective with `-E`) that triggers whenever a `module` directive is encountered where a _control-line_ or a _text-line_ is required.

In particular, such a diagnostic should only refer to "module directive"s and not to "module declaration"s.

A diagnostic is required for the following:
```cpp
module;
#if 0
export module m;
#endif
// expected-error at -2 {{}}
export module m2;
```

https://github.com/llvm/llvm-project/pull/107168