[dsymutil] Try to find lipo first besides dsymutil before looking up the PATH.

[dsymutil] Try to find lipo first besides dsymutil before looking up the PATH.

Even if we don't have it in PATH, lipo should usually exist in the same directory
as dsymutil. Keep the fallback looking up the PATH, it's very useful when
testing a non-installed executable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249762 91177308-0d34-0410-b5e6-96231b3b80d8