Align F18 supported build compilers to LLVM's

[RichBarton-Arm]

In order to become a part of the LLVM project we need to match the supported platforms and build compilers. There are some caveats to this:

• We require C++17 so the earliest supported compiler(s) may be too old. This has been discussed and is an acceptable situation for now.
• We don't have confidence enough to claim support on Windows yet. This is covered by Support Windows build compilers to match LLVM as closely as possible. #971

Here is what LLVM say about host platform and compiler support:

LLVM is very demanding of the host C++ compiler, and as such tends to expose bugs in the compiler. We also attempt to follow improvements and developments in the C++ language and library reasonably closely. As such, we require a modern host C++ toolchain, both compiler and standard library, in order to build LLVM.

LLVM is written using the subset of C++ documented in coding standards. To enforce this language version, we check the most popular host toolchains for specific minimum versions in our build systems:

Clang 3.5
Apple Clang 6.0
GCC 5.1
Visual Studio 2017

Anything older than these toolchains may work, but will require forcing the build system with a special option and is not really a supported host platform. Also note that older versions of these compilers have often crashed or miscompiled LLVM.

For less widely used host toolchains such as ICC or xlC, be aware that a very recent version may be required to support all of the C++ features used in LLVM.
https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library

Note also that there is a list of caveats for certain combinations of build compiler and standard C++ libraries, so we do have that option if we have particular problems.

To do:

• Understand which compilers can claim to support
• Update the Readme statement to match
• Create work items in https://github.com/orgs/flang-compiler/projects/10 to close the gaps.

[RichBarton-Arm]

From the flang community call on 19/02, we need to ensure that we can build F18 with the LLVM 10 RC before submitting to the monorepo.

[RichBarton-Arm]

Update in call on 09th March: @sscalpone has tested GCC 7.2+ on Power and x86 with no issues.
Now waiting for #969 to land before continuing on Clang versions.

[DavidTruby]

I believe this is done, as reflected on the project board. @RichBarton-Arm can you close this?

[RichBarton-Arm]

Agree that this is complete.