Sema: Predefine size_t in MSVC Compatibility mode

MSVC defines size_t without any explicit declarations.  This change
allows us to be compatible with TUs that depend on this declaration
appearing from nowhere.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199190 91177308-0d34-0410-b5e6-96231b3b80d8