Teach lit to filter the host LDFLAGS down from the build system and into

Teach lit to filter the host LDFLAGS down from the build system and into
the CGO build environment. This lets things like -rpath propagate down
to the C++ code that is built along side the Go bindings when testing
them.

Patch by Peter Collingbourne, and verified that it works by me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220252 91177308-0d34-0410-b5e6-96231b3b80d8