switch codebuild from clang 7 to clang 8

[kroening]

This is to catch problems as in #4744.

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

[tautschnig]

As noted on #4744, the Clang 8 build appears to be passing without changes. @kroening can you confirm that Clang 8 is genuinely being used in the build (I don't have access to the Clang 8 build log).

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 4a83428).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114238943

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (develop@d8598f8). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop    #4746   +/-   ##
==========================================
  Coverage           ?   68.44%           
==========================================
  Files              ?     1270           
  Lines              ?   104828           
  Branches           ?        0           
==========================================
  Hits               ?    71753           
  Misses             ?    33075           
  Partials           ?        0

---

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[kroening]

Yes, confirmed, cbmc-linux-clang builds with clang 8!

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 720e783).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114384475
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]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: d8cb021).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114386110
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: c3f2646).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114388107

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: f4451b5).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114459416

[peterschrammel]

CI passed