Windows: Fixed sys::findProgramByName to work with files containing dot in their name.

Problem was in SearchPathW function that does not attach an extension if file already has one.
That does not work for executables like ld.lld2 for example which require to have .exe extension but SearchPath thinks that its "lld2". 
Solution was to add the extension manually.

Differential Revision: http://reviews.llvm.org/D13536

llvm-svn: 249696