Use the -dumpversion option to llvm-gcc which gives us just the version #

Use the -dumpversion option to llvm-gcc which gives us just the version #
and is unlikely to change in future releases. This also simplifies the
parsing of the full and major llvm-gcc version numbers in the script.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28180 91177308-0d34-0410-b5e6-96231b3b80d8