[clang][deps] Split translation units into individual -cc1 or other commands

Instead of trying to "fix" the original driver invocation by appending
arguments to it, split it into multiple commands, and for each -cc1
command use a CompilerInvocation to give precise control over the
invocation.

This change should make it easier to (in the future) canonicalize the
command-line (e.g. to improve hits in something like ccache), apply
optimizations, or start supporting multi-arch builds, which would
require different modules for each arch.

In the long run it may make sense to treat the TU commands as a
dependency graph, each with their own dependencies on modules or earlier
TU commands, but for now they are simply a list that is executed in
order, and the dependencies are simply duplicated. Since we currently
only support single-arch builds, there is no parallelism available in
the execution.

Differential Revision: https://reviews.llvm.org/D132405