fix constructor of string_builtin_functiont

[kroening]

The default constructor cannot exist; marking it as delete re-enables
compilation with clang.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 5f94d92).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114010088

[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    #4744   +/-   ##
==========================================
  Coverage           ?   68.44%           
==========================================
  Files              ?     1270           
  Lines              ?   104828           
  Branches           ?        0           
==========================================
  Hits               ?    71753           
  Misses             ?    33075           
  Partials           ?        0

Impacted Files	Coverage Δ
src/solvers/strings/string_builtin_function.h	72.63% <ø> (ø)

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[tautschnig]

It’s still not clear to me why those aren’t caught in CI. We need to fix this while we still have a known-bad state.

[kroening]

I have looked into this.

clang-8 is stricter than clang-7 (but correctly so).

To catch these, we could switch the existing clang-7 build to clang-8. That will add a bit to the build time, and add some risk of that build failing, since the clang-8 binaries would need to be downloaded from apt.llvm.org

[tautschnig]

I am a bit worried, because #4746 appears to be passing even though this PR hasn't been merged.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: ef1737b).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114260970

[tautschnig]

I believe this has actually been merged as part of #4746 @kroening @peterschrammel ?! If so, please close this one.

[peterschrammel]

yes, has been merged.