Add clang.prependArgs option
To specify arguments that are inserted immediately after the compiler
driver name. For example, initialization options
`{"clang":{"prependArgs":["--gcc-install-dir=/usr/lib/gcc/x86_64-linux-gnu/13"]}}`
transforms `clang -c a.cc` into `clang --gcc-install-dir=/usr/lib/gcc/x86_64-linux-gnu/13 -c a.cc`.
This allows users to provide default arguments that can be overridden as
needed.