[ODRHash diagnostics] Transform method ASTReader::diagnoseOdrViolations into a class ODRDiagsEmitter. NFC.

Preparing to use diagnostics about ODR hash mismatches outside of ASTReader.

Differential Revision: https://reviews.llvm.org/D128490

Author: vsapsai

clang/include/clang/AST/DeclCXX.h

@@ -260,6 +260,7 @@ class CXXRecordDecl : public RecordDecl {
   friend class ASTWriter;
   friend class DeclContext;
   friend class LambdaExpr;
+  friend class ODRDiagsEmitter;
 
   friend void FunctionDecl::setPure(bool);
   friend void TagDecl::startDefinition();

clang/include/clang/Serialization/ASTReader.h

@@ -1845,10 +1845,6 @@ class ASTReader
   /// if the declaration is not from a module file.
   ModuleFile *getOwningModuleFile(const Decl *D);
 
-  /// Get the best name we know for the module that owns the given
-  /// declaration, or an empty string if the declaration is not from a module.
-  std::string getOwningModuleNameForDiagnostic(const Decl *D);
-
   /// Returns the source location for the decl \p ID.
   SourceLocation getSourceLocationForDeclID(serialization::GlobalDeclID ID);