[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.

tools/SourceKit/lib/SwiftLang/SwiftSourceDocInfo.cpp

@@ -61,7 +61,7 @@ static Type findBaseTypeForReplacingArchetype(const ValueDecl *VD, const Type Ty
 
   // Find the nominal type decl related to VD.
   NominalTypeDecl *NTD = VD->getDeclContext()->
-    isNominalTypeOrNominalTypeExtensionContext();
+    getAsNominalTypeOrNominalTypeExtensionContext();
   if (!NTD)
     return Type();
   Type Result;