[ASTContext] Add a templated convenience wrapper for Allocate.

This brings ASTContext closer to LLVM's Allocator concept. Ideally we
would just derive ASTContext from llvm::AllocatorBase, but that does
not work because ASTContext's allocator is mutable and we allocate using
const ASTContext& everywhere.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@243972 91177308-0d34-0410-b5e6-96231b3b80d8