pass around CompilerInstance rather than ASTContext

It's possible to get the latter from the former, and the former
is useful for other things too (access to the preprocessor, for example).

Change-Id: I708d709129fd3a35bf7c63da4de09c2e696b382d

compilerplugins/clang/unusedvariablecheck.cxx

@@ -30,14 +30,14 @@ SAL_WARN_UNUSED (see e.g. OUString). For external classes such as std::vector
 that cannot be edited there is a manual list below.
 */
 
-UnusedVariableCheck::UnusedVariableCheck( ASTContext& context )
-    : Plugin( context )
+UnusedVariableCheck::UnusedVariableCheck( CompilerInstance& compiler )
+    : Plugin( compiler )
     {
     }
 
 void UnusedVariableCheck::run()
     {
-    TraverseDecl( context.getTranslationUnitDecl());
+    TraverseDecl( compiler.getASTContext().getTranslationUnitDecl());
     }
 
 bool UnusedVariableCheck::VisitVarDecl( VarDecl* var )