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

[Michael Spencer via cfe-commits]

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@@ -1778,10 +1778,20 @@ def ext_bit_int : Extension<
 } // end of Parse Issue category.
 
 let CategoryName = "Modules Issue" in {
-def err_unexpected_module_decl : Error<
-  "module declaration can only appear at the top level">;
+def err_invalid_module_or_import_directive : Error<
+  "the %select{module|import}0 directive is ill-formed, "
+  "%select{module contextual keyword must be immediately "
+  "followed on the same line by an identifier, "
+  "or a ';' after being at the start of a line, or preceded by "
+  "an export keyword at the start of a line|"
+  "import contextual keyword must be immediately followed "
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Bigcheese wrote:

I don't think `module` and `import` are technically contextual keywords. Although maybe it's ok to not be technically correct here.

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