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