Bug 1479232 - Only use one matcher for varDecl and parmVarDecl r=andi

ParmVarDecl being a subclass of VarDecl, using two matchers then caused
ScopeChecker::check to be called twice for ParmVarDecl nodes, once for
each match. But the code in ScopeCheck::check is written with the
assumption that it's called only once for such nodes.

Somehow, this didn't cause problems with clang up to version 6, but
makes the plugin spuriously warn about already_AddRefed not being used
as temporaries when used as argument in function declarations, with
clang 7.

Differential Revision: https://phabricator.services.mozilla.com/D6360

--HG--
extra : moz-landing-system : lando

build/clang-plugin/ScopeChecker.cpp

@@ -11,7 +11,6 @@ void ScopeChecker::registerMatchers(MatchFinder *AstMatcher) {
   AstMatcher->addMatcher(materializeTemporaryExpr().bind("node"), this);
   AstMatcher->addMatcher(
       callExpr(callee(functionDecl(heapAllocator()))).bind("node"), this);
-  AstMatcher->addMatcher(parmVarDecl().bind("parm_vardecl"), this);
 }
 
 // These enum variants determine whether an allocation has occured in the code.
@@ -37,7 +36,7 @@ void ScopeChecker::check(const MatchFinder::MatchResult &Result) {
   QualType T;
 
   if (const ParmVarDecl *D =
-          Result.Nodes.getNodeAs<ParmVarDecl>("parm_vardecl")) {
+          Result.Nodes.getNodeAs<ParmVarDecl>("node")) {
     if (D->hasUnparsedDefaultArg() || D->hasUninstantiatedDefaultArg()) {
       return;
     }