Drop the ASTContext.h include from Stmt.h and fix up transitive users.

This required moving the ctors for IntegerLiteral and FloatingLiteral out of
line which shouldn't change anything as they are usually called through Create
methods that are already out of line.

ASTContext::Deallocate has been a nop for a long time, drop it from ASTVector
and make it independent from ASTContext.h

Pass the StorageAllocator directly to AccessedEntity so it doesn't need to
have a definition of ASTContext around.

llvm-svn: 159718