[HIP] Fix default output file for -E

By convention the default output file for -E is "-" (stdout).
This is expected by tools like ccache, which uses output
of -E to determine if a file and its dependence has changed.

Currently clang does not use stdout as default output file for -E
for HIP, which causes ccache not working.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D88730