Not all builds name the compiler executable `clang`. For example, the
Fuchsia build bots use `llvm` as their single toolchain executable name,
as they combine everything together in a busybot-style binary.
Update the Clang driver test to simply check that a non-empty path is
provided for the `--thinlto-remote-compiler` argument, rather than
hardcoding the executable name. The cross-project test will verify that
the path is valid later.
Should fix#147265.
edfec9c582