clang-cl: Alias /Zi and /Z7 to -gline-tables-only

LLVM only knows how to emit Z7-style line tables on -win32, so there's
no reason for clang to emit anything other than line info.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201017 91177308-0d34-0410-b5e6-96231b3b80d8