Use the *current* state of "is-moduleness" rather than the state at
serialization time so that if we read a builtin identifier from a module
that wasn't "interesting" to that module, we will still write it out to
a PCH that imports that module.
Otherwise, we would get mysterious "unknown builtin" errors when using
PCH+modules.
rdar://problem/23287656
llvm-svn: 251565
Ben Langmuir