[clang][AST][NFC] Remove a local variable

The reference could be const, but this is even better.

Author: tbaederr

clang/lib/AST/Decl.cpp

@@ -2200,8 +2200,7 @@ static LanguageLinkage getDeclLanguageLinkage(const T &D) {
 
   // Language linkage is a C++ concept, but saying that everything else in C has
   // C language linkage fits the implementation nicely.
-  ASTContext &Context = D.getASTContext();
-  if (!Context.getLangOpts().CPlusPlus)
+  if (!D.getASTContext().getLangOpts().CPlusPlus)
     return CLanguageLinkage;
 
   // C++ [dcl.link]p4: A C language linkage is ignored in determining the