- use major-only for Clang >= 8.0

[SSE4]

Changelog: Feature: Added support for Clang >=8 and the new versioning schema, where only the major and the patch is used.

according to the:
https://releases.llvm.org/7.0.0/tools/clang/docs/ReleaseNotes.html

Clang binary and libraries have been renamed from 7.0 to 7. For example, the clang binary will be called clang-7 instead of clang-7.0.

also according to the:
http://lists.llvm.org/pipermail/llvm-dev/2018-March/121873.html

Now, as we are not using the minor version (the Y in X.Y.Z), there isn't
much point in calling our tools foo-7.0 as we won't create a 7.1.

and finally:
http://blog.llvm.org/2016/12/llvms-new-versioning-scheme.html

We decided that going forward, every release on the six-month cycle will be a major release. Patch releases will increment the patch component as before (producing versions like 5.0.1), and the minor component will stay at zero since no minor releases will be made.

so, this patch offers to use major only versions for Clang >= 7.0, similarly on how it is done for GCC >= 5.0

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one. Also adding a description of the changes in the changelog.rst file. https://github.com/conan-io/docs

[SSE4]

build problem appears to be an infrastructure issue:

ERROR: Issue with creating launcher for agent MacosBuilder. The agent has not been fully initialized yet

[SSE4]

same question, do we have such quirks for GCC? also, how about patch versions for GCC? I guess they currently don't matter, assuming all patch versions are compatible, if major is same, right?
so, I wonder, why Clang logic must be different from GCC's? or do you mean it should be changed for both GCC and Clang?

[SSE4]

updated, summary:

• added Clang 8 to settings.yml
• starting from Clang 8, conan will detect major-only by default
• detection for Clang 7 and below is not changed to avoid compatibility breakage
• added major-only Clang versions 5, 6, 7, 8 to settings.yml
• migrations.py is not changed
• added missing compilers to the new_ci.py (GCC 8, Clang 5, 6, 7, Apple Clang 10.0)
• CMake-wrapper checks major-only since Clang 8 as well

[Croydon]

LGTM

[lasote]

@memsharded please confirm that we are following this approach of keeping clang 7.0 and merge when you are ready.