[Clang] Allow processing of attributes on statements by plugins

[Qix-]

Summary

Pretty cut and dry; currently plugins can create attributes for declarations but any that are not recognized produce a diagnostic, leaving no hook for plugins to react to them.

This adds a quick check to the attribute to see if the implementation would like to handle it first, akin to how declarations are handled.

Compatible with D99861.

[aaron.ballman]

Thank you for the patch! Btw, can you add more context to the patch when you generate it (I usually use -U 999 when making patches)?

The changes look good so far, but I think docs\ClangPlugins.rst should be updated to document the new functionality and it would be nice to add example usage (in lieu of tests) to examples\Attribute.

[Qix-]

Closing in favor of more complete plugin attribute changes discussed in IRC.

clang/include/clang/Sema/ParsedAttr.h

struct ParsedAttrInfo {
   };
   /// If this ParsedAttrInfo knows how to handle this ParsedAttr applied to this
   /// Decl then do so and return either AttributeApplied if it was applied or
   /// AttributeNotApplied if it wasn't. Otherwise return NotHandled.
   virtual AttrHandling handleDeclAttribute(Sema &S, Decl *D,
                                            const ParsedAttr &Attr) const {
     return NotHandled;
   }
+  /// If this ParsedAttrInfo knows how to handle this ParsedAttr applied to this
+  /// Stmt then do so and return either AttributeApplied if it was applied or
+  /// AttributeNotApplied if it wasn't. Otherwise return NotHandled.
+  virtual AttrHandling handleStmtAttribute(Sema &S, Stmt *St,
+                                           const ParsedAttr &Attr) const {
+    return NotHandled;
+  }
 
   static const ParsedAttrInfo &get(const AttributeCommonInfo &A);
 };
 
 typedef llvm::Registry<ParsedAttrInfo> ParsedAttrInfoRegistry;
 
 /// Represents information about a change in availability for
 /// an entity, which is part of the encoding of the 'availability'

clang/lib/Sema/SemaStmtAttr.cpp

case ParsedAttr::AT_Suppress:
     return handleSuppressAttr(S, St, A, Range);
   case ParsedAttr::AT_NoMerge:
     return handleNoMergeAttr(S, St, A, Range);
   case ParsedAttr::AT_Likely:
     return handleLikely(S, St, A, Range);
   case ParsedAttr::AT_Unlikely:
     return handleUnlikely(S, St, A, Range);
   default:
+    if (A.getInfo().handleStmtAttribute(S, St, A) == ParsedAttrInfo::NotHandled)
       // N.B., ClangAttrEmitter.cpp emits a diagnostic helper that ensures a
       // declaration attribute is not written on a statement, but this code is
       // needed for attributes in Attr.td that do not list any subjects.
       S.Diag(A.getRange().getBegin(), diag::err_decl_attribute_invalid_on_stmt)
           << A << St->getBeginLoc();
     return nullptr;
   }
 }
 
 StmtResult Sema::ProcessStmtAttributes(Stmt *S,
                                        const ParsedAttributesView &AttrList,
                                        SourceRange Range) {
   SmallVector<const Attr*, 8> Attrs;