Fix detection of COFF executable files.

One overload of this function would try to identify a file
by opening it and using the first 32 bytes to identify the magic
of the file.  This didn't work properly when more than 32 bytes
is actually needed for magic detection to succeed.  So now we
have this overload read in the entire file.

Differential Revision: https://reviews.llvm.org/D44225

llvm-svn: 327050