[llvm-rc] Allow -1 for control IDs in old style dialogs with 16 bit fields

-1 is commonly used as ID for controls that one don't want to
refer to later. For DIALOG resources, the IDs are 16 bit numbers,
and -1 gets interpreted as UINT32_MAX earlier, which then later is
too large to write into a uint16_t.

Differential Revision: https://reviews.llvm.org/D46506

llvm-svn: 331901