PCH versus --enable-mapped-location

[Per Bothner]

Daniel Jacobowitz wrote:
  > That's exactly what Geoff said.  There are two relevant properties of
> GCed memory here:
>   - Anything in GCed memory will be saved to the PCH
>   - Anything in GCed memory will be overwritten by loading the PCH.

So the corrollary: After a restore any pointers from non-gc'd memory to
gc'd memory will be a dangling pointer, if we count static variables 
marked with GTY as "gc'd memory" in this context.

Right now, as far as I can tell, the filenames in the line_table are
allocated from non-gc'd memory.  This complicates using gc.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/