Additional `SOURCE_FILENAME` variable in CppCompileVariables

### Description of the feature request:

We currently define `__SHORT_FILENAME__=basename of source_file` in cmake using `COMPILE_DEFINITIONS` feature. Currently, I don't see any way to define that flag inside of bazel. I tried

```
# pseudo code
flag_group('-D__SHORT_FILENAME__=$(basename %{source_file})')
```
but that fails to work because it gets escaped during the shell expansion with single quotes to `'-D__SHORT_FILENAME=$(basename path_to_file/file.cpp)'`.

So, would it be okay for me to make a change to the `CompileBuildVariables.java` and add a enum `SOURCE_FILENAME` that just stores the name of the file.

Start by reading CompileBuildVariables.java and the existing CMake COMPILE_DEFINITIONS handling described in the issue. Trace how CppCompileVariables are exposed to Bazel flag expansion, then verify that the requested SOURCE_FILENAME value is available as the source file name without shell escaping; done means the variable can support the stated compile-definition use case.

Written by the indexing model from the issue text.