PR 14400--PCH problem with large files

[Ian Lance Taylor]

Geoff, could you take a look at PR 14400?  It is a PCH problem which
arises with large files.  Basically, gcc currently reads the entire
source file into memory before it loads any PCH (in read_file_guts()
called eventually by cpp_read_main_file()).  I think this means that
on a system for which PCH requires that mmap generally return the same
address--i.e., any non-Darwin system--PCH will fail for a sufficiently
large source file.

I'm a bit tired, so I may be missing something.  If not, I think we
should add a test to the PCH testsuite which tries to compile a large
file.  I'm not sure what a good fix would be.

Ian