Fix windows-nat.c for -Wnarrowing

Fix windows-nat.c for -Wnarrowing

Sergio pointed out that the Windows builder was failing due to the
-Wnarrowing patch, with:

../../binutils-gdb/gdb/windows-nat.c:301:27: error: narrowing conversion of '3221225477' from 'DWORD {aka long unsigned int}' to 'int' inside { } [-Wnarrowing]
   {-1, GDB_SIGNAL_UNKNOWN}};
                           ^
../../binutils-gdb/gdb/windows-nat.c:301:27: error: narrowing conversion of '3221225725' from 'DWORD {aka long unsigned int}' to 'int' inside { } [-Wnarrowing]
../../binutils-gdb/gdb/windows-nat.c:301:27: error: narrowing conversion of '2147483651' from 'DWORD {aka long unsigned int}' to 'int' inside { } [-Wnarrowing]
../../binutils-gdb/gdb/windows-nat.c:301:27: error: narrowing conversion of '2147483652' from 'DWORD {aka long unsigned int}' to 'int' inside { } [-Wnarrowing]
../../binutils-gdb/gdb/windows-nat.c:301:27: error: narrowing conversion of '3221225614' from 'DWORD {aka long unsigned int}' to 'int' inside { } [-Wnarrowing]

Looking into this, I found two things.

First, in struct xlate_exception, it is better to have "them" be of
type DWORD, as that's the type actually in use.

Second, struct xlate_exception and xlate are not used in this file,
because the code in windows_nat_target::resume is #if'd out.

This patch changes the type of "them", but also similarly #if's out
this object.

In order to avoid a narrowing warning from the -1 entry, at Pedro's
suggestion I have removed this and changed windows_nat_target::resume
to use ranged for.

Tested by rebuilding using the mingw toolchain on x86-64 Fedora 28.  I
also tested it by temporarily removing the "#if 0"s and rebuilding.

gdb/ChangeLog
2018-08-29  Tom Tromey  <tom@tromey.com>

	* windows-nat.c (struct xlate_exception) <them>: Change type to
	DWORD.
	(xlate): Fix formatting.  Remove last entry.
	(struct xlate_exception, xlate): Comment out.
	(windows_nat_target::resume): Use ranged for.