[driver] Perform fallback target searches for stdlib

Searching for target-specific standard library header and library paths
should perform fallback searches for targets, the same way searching for
the runtime libraries does. It's important for the header and library
searches to be consistent, otherwise we could end up using mismatched
headers and libraries. (See also https://reviews.llvm.org/D146664.)

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D159293