[AST] rename some isXXX methods to getAsXXX

There's a group of methods in `DeclContext` with names that start with *is*,
such as `isClassOrClassExtensionContext()`. These names suggests a boolean
return value, while the methods actually return a type declaration. This
patch replaces the *is* prefix with *getAs* to better reflect their interface.

lib/Sema/DerivedConformanceEquatableHashable.cpp

@@ -302,7 +302,7 @@ deriveBodyHashable_enum_hashValue(AbstractFunctionDecl *hashValueDecl) {
   auto parentDC = hashValueDecl->getDeclContext();
   ASTContext &C = parentDC->getASTContext();
 
-  auto enumDecl = parentDC->isEnumOrEnumExtensionContext();
+  auto enumDecl = parentDC->getAsEnumOrEnumExtensionContext();
   SmallVector<ASTNode, 3> statements;
   auto selfDecl = hashValueDecl->getImplicitSelfDecl();