Add an autoconf option for turning on -gsplit-dwarf by default
when building llvm. This saves quite a bit of time and space when
linking. Please report any problems via bugzilla.

Caveats:

a) This will only work on linux
b) This requires a fairly new binutils
c) This requires a fairly new gdb

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184808 91177308-0d34-0410-b5e6-96231b3b80d8