[clang][deps] Store common, partially-formed invocation (#65677)

We create one `CompilerInvocation` for each modular dependency we
discover. This means we create a lot of copies, even though most of the
invocation is the same between modules. This patch makes use of the
copy-on-write flavor of `CompilerInvocation` to share the common parts,
reducing memory usage and speeding up the scan.