_swift_withWin32DbgHelpLibrary() should not use GetCurrentProcess()

[grynspan]

Description

The Microsoft documentation for the DbgHelp library warns developers not to pass GetCurrentProcess() as the handle to SymInitialize() etc. We should use some other handle, e.g. one returned from GlobalAlloc().

I'll modify _swift_withWin32DbgHelpLibrary() to use a unique handle and pass it to its callback.

Steps to reproduce
N/A

Expected behavior
N/A

Environment
N/A