[clangd] When querying drivers by binary, look in PATH too

Sometimes compile_commands.json databases are created without an
absolute path for the driver in the command field. By default the driver
name is appended to the current directory, however if no driver is found
in that location assume it was in the default PATH and try finding it
there

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D93600