Serialization: use the PCH chain to check PCH mode

When we are deciding whether we are creating a PCH or a module, we would
check if the ModuleMgr had any elements to switch into PCH mode.
However, when creating a module, the size may be 1.  This would result
in us going down the wrong path.

This was found by cross-compiling the swift standard library.  Use the
PCH chain length instead to identify the PCH mode.

Unfortunately, I have not yet been able to create a simple test case for
this, but have verified that this fixes the swift standard library
construction.

Thanks to Adrian Prantl for help and discussions with this change!

llvm-svn: 296769