Part of http://reviews.llvm.org/D7827. This converts most of WinSymbolizer into WinSymbolizerTool, which uses the common SymbolizerTool. To make the changes more smooth (and patches more readable), I didn't mess up with POSIXSymbolizer, yet. But eventually, I want to "upgrade" POSIXSymbolizer into a cross-platform Symbolizer, that would use WinSymbolizer just as any other symbolizer tool.
For the transition, I made a small class that does the same as POSIXSymbolizer but for Windows.
One issue with the WinSymbolizer is that is uses DbgHelp even for finding module name and offset from an address. I made it a static function, which also needed the initialization functions to be static and the initialized_ field global. The other option would be to add the FindModuleNameAndOffsetForAddress into the SymbolizerTool interface, which however doesn't sound right to me: On POSIX, all the symbolizer tools use the same way to finding module names, so it looks like it's more of a platform-specific thing than a tool-specific one.