Add CI compilation with clang++-3.8

[romainbrenguier]

This is to check that changes do not break compilation using
clang++-3.8, as happened with
#4699

• Each commit message has a non-empty body, explaining why the change was made.
• [na] Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• [na] The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• [na] Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• [na] My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• [na] White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

@kroening @romainbrenguier @peterschrammel Could this be moved into a new CodeBuild job instead to be properly run in parallel?

[peterschrammel]

@kroening @romainbrenguier @peterschrammel Could this be moved into a new CodeBuild job instead to be properly run in parallel?

Yes, please move that into a new buildspec-linux-clang-3.8.yml file.

[romainbrenguier]

@peterschrammel @tautschnig I added a new buildspec-linux-clang-3.8.yml file but it looks like only buildspec.yml and buildspec-windows.yml are run on CI

[peterschrammel]

only buildspec.yml and buildspec-windows.yml are run on CI

I'll add that to codebuild

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 7b1280f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113845577

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 872198f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113875367
Status will be re-evaluated on next push.
Common spurious failures include: the cbmc commit has disappeared in the mean time (e.g. in a force-push); the author is not in the list of contributors (e.g. first-time contributors); compatibility was already broken by an earlier merge.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 88abf60).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113876083

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 7ef487a).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114072123

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 45fc621).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114079433

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (develop@ba23989). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop    #4718   +/-   ##
==========================================
  Coverage           ?   68.46%           
==========================================
  Files              ?     1270           
  Lines              ?   104777           
  Branches           ?        0           
==========================================
  Hits               ?    71740           
  Misses             ?    33037           
  Partials           ?        0

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