[PATCH] Make lto-dump dump symtab callgraph in graphviz format

[Giuliano Belinassi]

This patch makes lto-dump dump the symbol table callgraph in graphviz
format.

I've not added any test to this because I couldn't find a way to call
lto-dump from the testsuite. Also, any feedback with regard to how can
I improve this is welcome.

    gcc/ChangeLog
    2019-06-29  Giuliano Belinassi  <giuliano.belinassi@usp.br>

            * cgraph.c (dump_graphviz): New function
            * cgraph.h (dump_graphviz): New function
            * symtab.c (dump_graphviz): New function
            * varpool.c (dump_graphviz): New function

    gcc/lto/ChangeLog
    2019-06-29  Giuliano Belinassi  <giuliano.belinassi@usp.br>

            * lang.opt (flag_dump_callgraph): New flag
            * lto-dump.c (dump_symtab_graphviz): New function
            * lto-dump.c (dump_tool_help): New option
            * lto-dump.c (lto_main): New option