[clang][ScanDeps] Fix issue with multiple commands with the same input.

Previously, given a CompilationDatabase with two commands for the same
source file we would report that file twice with the union of the
dependencies for each command both times.

This was due to the way `ClangTool` runs actions given an input source
file (see the comment in `DependencyScanningTool.cpp`). This commit adds
a `SingleCommandCompilationDatabase` that is created with each
`CompileCommand` in the original CDB, which is then used for each
`ClangTool` invocation. This gives us a single run of
`DependencyScanningAction` per `CompileCommand`.

I looked at using `AllTUsToolExecutor` which is a parallel tool
executor, but I'm not sure it's suitable for `clang-scan-deps` as it
does a lot more sharing of state than `AllTUsToolExecutor` expects.

Differential Revision: https://reviews.llvm.org/D69643