Fix missing builtin identifier infos with PCH+modules

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