Make the deserialization of macro definitions lazy, so that we can

Make the deserialization of macro definitions lazy, so that we can
load identifiers without loading their corresponding macro
definitions. This is likely to improve PCH performance slightly, and
reduces deserialization stack depth considerably when using
preprocessor metaprogramming.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@117750 91177308-0d34-0410-b5e6-96231b3b80d8