[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/Parse/ParsePattern.cpp

@@ -784,7 +784,7 @@ ParserResult<Pattern> Parser::parsePattern() {
   }
     
   case tok::code_complete:
-    if (!CurDeclContext->isNominalTypeOrNominalTypeExtensionContext()) {
+    if (!CurDeclContext->getAsNominalTypeOrNominalTypeExtensionContext()) {
       // This cannot be an overridden property, so just eat the token. We cannot
       // code complete anything here -- we expect an identifier.
       consumeToken(tok::code_complete);