[llvm-rc] Use proper search algorithm for finding resources.

Previously we would only look in the current directory for a
resource, which might not be the same as the directory of the
rc file.  Furthermore, MSVC rc supports a /I option, and can
also look in the system environment.  This patch adds support
for this search algorithm.

Differential Revision: https://reviews.llvm.org/D38740

llvm-svn: 315499