Address code review.

Author: qiongsiwu

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -909,7 +909,8 @@ def err_mmap_nested_submodule_id : Error<
   "qualified module name can only be used to define modules at the top level">;
 def err_mmap_expected_feature : Error<"expected a feature name">;
 def err_mmap_expected_attribute : Error<"expected an attribute name">;
-def err_mmap_link_redecalration : Error<"redeclaration of link library '%0'">;
+def warn_mmap_link_redeclaration : Warning<"redeclaration of link library '%0'">,
+  InGroup<DiagGroup<"module-link-redeclaration">>, DefaultError;
 def note_mmap_prev_link_declaration : Note<"previously declared here">;
 def err_mmap_submodule_link_decl
     : Error<"link declaration is not allowed in submodules">;

clang/lib/Lex/ModuleMapFile.cpp

@@ -864,7 +864,7 @@ std::optional<LinkDecl> ModuleMapFileParser::parseLinkDecl(
   auto [It, Inserted] =
       SeenLinkDecl.insert(std::make_pair(Library, LD.Location));
   if (!Inserted) {
-    Diags.Report(LD.Location, diag::err_mmap_link_redecalration) << Library;
+    Diags.Report(LD.Location, diag::warn_mmap_link_redeclaration) << Library;
     Diags.Report(It->second, diag::note_mmap_prev_link_declaration);
     HadError = true;
     return std::nullopt;