Update Preprocessor::Initialize() documentation following r207825

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207895 91177308-0d34-0410-b5e6-96231b3b80d8
Fix a bunch of mislayered clang/Lex includes from Sema

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207896 91177308-0d34-0410-b5e6-96231b3b80d8
Eliminate ASTContext's DelayInitialization flag

Having various possible states of initialization following construction doesn't
add value here.

Also remove the unused size_reserve parameter.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207897 91177308-0d34-0410-b5e6-96231b3b80d8