Reloc.SymbolTableIndex is an index into symbol table. But
Reloc.SymbolTableIndex cannot be used as index into File.Symbols,
because File.Symbols slice has Aux lines removed as it is built.

We cannot change the way File.Symbols works, so I propose we
introduce new File.COFFSymbols that does not have that limitation.

Also unlike File.Symbols, File.COFFSymbols will consist of
COFFSymbol. COFFSymbol matches PE COFF specification exactly,
and it is simpler to use.

Updates #15345

Change-Id: Icbc265853a472529cd6d64a76427b27e5459e373
Reviewed-on: https://go-review.googlesource.com/22336Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>