COFF: Use a bit in SymbolBody to track which symbols are written to the symbol table.

Using a set here caused us to take about 1 second longer to write the symbol
table when linking chrome_child.dll. With this I consistently get better
performance on Windows with the new symbol table.

Before r289280 and with r289183 reverted (median of 5 runs): 17.65s
After this change: 17.33s

On Linux things look even better:

Before: 10.700480444s
After: 5.735681610s

Differential Revision: https://reviews.llvm.org/D27648

llvm-svn: 289408