[WIP] MSVC tool: prevent overwriting the PCH object node builder/emitter

[jcbrill]

Prevent overwriting the PCH object node builder/emitter when the PCH source file is included in the source list.

Suppress the assignment of the static object builder/emitter when the target is the PCH object file (e.g., PCH.obj) and the source is the PCH c++ file (e.g., PCH.cpp).

When the object_emitter is called for target PCH.obj and source PCH.cpp, the target list and source list are invalidated (i.e., both are set to None).

See https://github.com/SCons/scons/issues/2249#issuecomment-1802157530 for discussion.

Contributor Checklist:

• [x] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [ ] I have updated CHANGES.txt (and read the README.rst)
• [ ] I have updated the appropriate documentation