[Windows] Implement PrintStackTrace(FILE*)

llvm::sys::PrintBacktrace(FILE*) is supposed to print a backtrace
of the current thread given the current PC.  This function was
unimplemented on Windows, and instead the only time we could
print a backtrace was as the result of an exception through
LLVMUnhandledExceptionFilter.

This patch implements backtracing of self by using
RtlCaptureContext to get a CONTEXT for the current thread, and
moving the printing and StackWalk64 code to a common method that
printing own stack trace and printing stack trace of an exception
can use.

Differential Revision: http://reviews.llvm.org/D8068
Reviewed by: Reid Kleckner

llvm-svn: 231382