Extend the ASTContext constructor to delay the initialization of

Extend the ASTContext constructor to delay the initialization of
builtin types (When requested). This is another step toward making
ASTUnit build the ASTContext as needed when loading an AST file,
rather than doing so after the fact. No actual functionality change (yet).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138985 91177308-0d34-0410-b5e6-96231b3b80d8