Lazily load the controlling macros for all of the headers known in the

Lazily load the controlling macros for all of the headers known in the
PCH file. In the Cocoa-prefixed "Hello, World" benchmark, this takes
us from reading 503 identifiers down to 37 and from 470 macros down to
4. It also results in an 8% performance improvement.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70094 91177308-0d34-0410-b5e6-96231b3b80d8