[LTO] Make Move lto-symtab.c out of lto/

[Doug Kwan (關振德)]

Rafael,

     I have a sandbox for making ipa-cp.c WHOPR friend but I ran into
problem as some of the lto_symtab functions are used indirectly and I
got linker errors in the C/C++ FEs because the lto_symbtab functions
are into the lto1 FE only.  Although we do not read IR files in
non-lto FEs, the IR reading functions are present in all FEs due to
pointers to them from the ipa_opt_pass structs.   The summary readers
now need to know about symbol resolution to discard or ignore summary
information.  For example, you had to extract a part of the
lto-cgraph.c into lto1 because it needs to use
lto_symtab_prevailing_decl.   You also defined a lang-hook just for
input_cgraph.  I don't think we should define other lang-hooks for
other IPA summary readers.  Instead the lto_symtab functions should be
visibile to all FEs. This can simply be done by pulling the file
lto-symtab.c out of lto1.  When that it done.  I think we can
eliminate the input_cgraph hook and move back the code to
lto-cgraph.c, where it logically belongs.

Thoughts? Comments?

-Doug