Two changes to Sema::LookupDecl() interface.

Two changes to Sema::LookupDecl() interface.
(1) Remove IdLoc (it's never used).
(2) Add a bool to enable/disable lazy builtin creaation (defaults to true).

This enables us to use LookupDecl() in Sema::isTypeName(), which is also part of this commit.

To make this work, I changed isTypeName() to be a non-const member function. I'm not happy with this, however I fiddled with making LookupDecl() and friends const and it got ugly pretty quickly. We can certainly add it back if/when someone has time to fiddle with it. For now, I thought this simplification was more important than retaining the const-ness.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@49087 91177308-0d34-0410-b5e6-96231b3b80d8