llvm-build: Get rid of 'import *'

This allows pyflakes catching more errors in the script.

Differential Revision: http://reviews.llvm.org/D3334



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207012 91177308-0d34-0410-b5e6-96231b3b80d8