[ELF] Dump symbols ordered by profiled guided section layout to file.

Patch by Tiancong Wang.

In D36351, Call-Chain Clustering (C3) heuristic is implemented with
option --call-graph-ordering-file <file>.
This patch adds a flag --print-symbol-order=<file> to LLD, and when
specified, it prints out the symbols ordered by the heuristics to the
file. The symbols printout is helpful to those who want to understand
the heuristics and want to reproduce the ordering with
--symbol-ordering-file in later pass.

Differential Revision: https://reviews.llvm.org/D59311

llvm-svn: 357133