PCH: Do not mark PCH-header as generated

The PCH header file itself is written by CMake and is not generated by part of the buildsystem. Therefore the GENERATED property is not appropriate to set because the file is always present before the build starts. We already do not mark the PCH source file as GENERATED either.

This uses solution 1 described in the "Conclusion" section of #21437 (closed)

Note:

This is a preparation for solving #18399 (closed), which will no longer allow to unset the GENERATED property from a source-file once it was set.

Blocks: !5308 (merged)
Fixes: #21437 (closed)
Topic-rename: pch-header-not-generated