WIP: Add Fastbuild generator

Implements Fastbuild generator (#15294).

Needs the following Fastbuild changes:

• https://github.com/fastbuild/fastbuild/pull/671 Reusing PCH files across targets
• https://github.com/fastbuild/fastbuild/pull/681 Merged in the dev branch
• ExecAlwaysShowOutput in the dev branch (otherwise Fastbuild doesn't output command results)

Current limitations:

• the triggering of CMake regeneration is partially implemented, need to figure out the right way to implement it with fastbuild
• Fastbuild removes empty lines from the output of commands, breaks some tests
• Fastbuild cleans and builds at the same time, it can't do clean without building. This breaks some assumptions in CMake and some tests

TODO:

• Tests to be fixed:
  • RunCMake.ClangTidy - output mangling, expected :0:0 is converted to (0,0) (it might be just a versioning difference of clang-tidy?)
  • RunCMake.MultiLint - output mangling, expected :0:0 is converted to (0,0), also needs empty lines fix
  • RunCMake.ExternalProject - needs empty lines fix and standalone clean
  • RunCMake.ctest_labels_for_subprojects - hard to debug, not sure what is wrong
  • RunCMake.GoogleTest - it might be the empty line removal of Fastbuild, but it might be something else as well
  • Qt5Autogen.RerunRccDepend - needs CMake regeneration
  • RunCMake.Configure - needs CMake regeneration
  • RunCMake.file - needs CMake regeneration
• Test Linux
• Test Mac OSX

Future work/nice to have:

• distributed compilation
• change cache folder (so it can be shared)
• enable/disable cache
• reduce dependencies between different targets' ObjectLists to improve concurrency
• translate CMake unity builds to Fastbuild native support
• multiconfiguration support like was added to Ninja
• swift support

I have tested this version on Windows, still need to test Mac and Linux.