Having a build tree with "not" and "FileCheck" is still required, but if
Clang/Flang isn't configured in that build, run the tests with the same
compiler CMake uses. This is how testing worked in the standalone build
configurations that now have been removed.
Go into more detail on the two non-legacy build modes.
I decided use create a dedicated document documention the build process.
`index.rst` only keeps the "Getting Started" part. `README.rst` is
vastly outdated with the still valid parts integrated into the new
documentation. `SupportAndFAQ.rst` is unstructured and keeps only the
non-building parts.
The the new building document is written in Meltdown, following the
decision from LLVM.
