[flang] A rework of the cmake build components for in and out of tree builds.

[flang] A rework of the cmake build components for in and out of tree builds.

In general all the basic functionality seems to work and removes some redundancy
and more complicated features in favor of borrowing infrastructure from LLVM
build configurations. Here's a quick summary of details and remaining issues:

  * Testing has spanned Ubuntu 18.04 & 19.10, CentOS 7, RHEL 8, and
    MacOS/darwin.  Architectures include x86_64 and Arm.  Without
    access to Window nothing has been tested there yet.

  * As we change file and directory naming schemes (i.e.,
    capitalization) some odd things can occur on MacOS systems with
    case preserving but not case senstive file system configurations.
    Can be painful and certainly something to watch out for as any
    any such changes continue.

  * Testing infrastructure still needs to be tuned up and worked on.
    Note that there do appear to be cases of some tests hanging (on
    MacOS in particular).  They appear unrelated to the build
    process.

  * Shared library configurations need testing (and probably fixing).

  * Tested both standalone and 'in-mono repo' builds.  Changes for
    supporting the mono repo builds will require LLVM-level changes that
    are straightforward when the time comes.

  * The configuration contains a work-around for LLVM's C++ standard mode
    passing down into Flang/F18 builds (i.e., LLVM CMake configuration would
    force a -std=c++11 flag to show up in command line arguments.  The
    current configuration removes that automatically and is more strict in
    following new CMake guidelines for enforcing C++17 mode across all the
    CMake files.

  * Cleaned up a lot of repetition in the command line arguments.  It
    is likely that more work is still needed to both allow for
    customization and working around CMake defailts (or those
    inherited from LLVM's configuration files). On some platforms agressive
    optimization flags (e.g. -O3) can actually break builds due to the inlining
    of templates in .cpp source files that then no longer are available for use
    cases outside those source files (shows up as link errors).   Sticking at -O2
    appears to fix this.  Currently this CMake configuration forces this in
    release mode but at the cost of stomping on any CMake, or user customized,
    settings for the release flags.

  * Made the lit tests non-source directory dependent where appropriate. This is
    done by configuring certain test shell files to refer to the correct paths
    whether an in or out of tree build is being performed. These configured
    files are output in the build directory. A %B substitution is introduced in
    lit to refer to the build directory, mirroring the %S substitution for the
    source directory, so that the tests can refer to the configured shell scripts.

Co-authored-by: David Truby <david.truby@arm.com>
Original-commit: flang-compiler/f18@d1c7184159b2d3c542a8f36c58a0c817e7506845
Reviewed-on: https://github.com/flang-compiler/f18/pull/1045