[ELF] Initialize 2 fields of Symbol in SymbolTable::insert

A new symbol is added to elf::symtab in 3 steps:

1) SymbolTable::insert creates a placeholder.
2) Symbol::mergeProperties
3) Symbol::replace

Fields referenced by steps 2) and 3) should be initialized in
SymbolTable::insert.  `traced` and `referenced` were missed previously.
This did not cause problems because compilers generated code that
initialized them (bit fields) to 0.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D66130

llvm-svn: 368784