[llvm-rc] Read the Planes/BitCount fields from BITMAPINFOHEADER for icons

Previously these fields were only read from this header for cursors,
while Planes was hardcoded to 1 for icons (with a comment that it was
unknown why this was needed) and BitCount was left at the value
read originally in the RESDIRENTRY.

This fixes the single byte that was differing for the icon/cursor test
compared to rc.exe.

This is based on research/testing by Nico Weber.

Differential Revision: https://reviews.llvm.org/D46816

llvm-svn: 332328