Limit the lenght of the file name of the log file for tests

If a test have very long name and the compiler specified with (a long)
full path then the name of the log file name can exceed 255 characters.
This change replace the full compiler path with just the compiler name
if the prior would cause a too long file name.

Differential revision: http://reviews.llvm.org/D8252

llvm-svn: 232024