Simplify mutex handling here, use more RAII

That required improving UnusedVariableCheck to honor [[maybe_unused]].

Change-Id: I01f3fd92d78726582f873681c807315b029c4e89
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/200032
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Jenkins

compilerplugins/clang/test/unusedvariablecheck.cxx

@@ -33,6 +33,7 @@ int main()
         v4; // expected-error {{unused variable 'v4' [loplugin:unusedvariablecheck]}}
     S v5; // expected-error {{unused variable 'v5' [loplugin:unusedvariablecheck]}}
     S v6(nullptr);
+    [[maybe_unused]] S v7;
 }
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */

compilerplugins/clang/unusedvariablecheck.cxx

@@ -50,7 +50,7 @@ bool UnusedVariableCheck::VisitVarDecl( const VarDecl* var )
     {
     if( ignoreLocation( var ))
         return true;
-    if( var->isReferenced() || var->isUsed())
+    if( var->isReferenced() || var->isUsed() || var->hasAttr<UnusedAttr>())
         return true;
     if( var->isDefinedOutsideFunctionOrMethod())
         return true;