[flang][driver] Unify f18_version.h.in and Version.inc.in

[flang][driver] Unify f18_version.h.in and Version.inc.in

Flang has two CMake configurable header files that define compiler
version numbers:
* f18_version.h.in - only used in f18.cpp (uses version numbers from
  LLVM's macro definitions)
* Version.inc.in - not currently used (uses version numbers hard-coded
  in Flang's top CMake script)

Currently only f18_version.h.in provides version numbers consistent with
other subprojects in llvm-project. However, its location and name are
inconsistent with e.g. Clang. This patch merges the two headers
together:
  * hard-coded version numbers in Flang's top CMake script are deleted
  * Version.inc.in is updated to provide string versions of version
  numbers (required by f18.cpp)
  * f18_version.h.in is deleted as it's no longer needed

Differential Revision: https://reviews.llvm.org/D94422