[AST] Add missing const qualifiers to AstContext in Type.cpp

Summary:
Add some missing const qualifiers to AstContext.
The ASTContext can't be modified with accessors.

There is no behavior change. This patch is cleanup only.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20226

llvm-svn: 269418