travis: Run unit tests --with-sanitizers=undefined #14036

See issue #12691

<!--e57a25ab6845829454e8d69fc972939a-->Note to reviewers: This pull request conflicts with the following ones:

• #13863 (travis: move script sections to files in .travis/ subject to shellcheck by scravy)
• #13827 (wip, depends: Add native_nsis to support unicode by ken2812221)
• #13515 (travis: avoid timeout without saving caches, also enable all qt by ken2812221)
• #12134 (Build previous releases and run functional tests by Sjors)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

This is excellent!

Strongest possible concept ACK (assuming we’re not hitting build timeout issues blocking this – that the builds take too long to finish)

Concept ACK

Concept ACK

From the build log:

checking whether we are using Intel C compiler... no
checking whether we are using Sun Studio C compiler... no
checking whether we are using clang C compiler... no
checking whether we are using gcc >= 4 C compiler... yes
checking whether the C++ compiler works... yes
checking whether we are using Intel C++ compiler... no
checking whether we are using Sun Studio C++ compiler... no
checking whether we are using clang C++ compiler... no
checking whether we are using gcc >= 4 C++ compiler... yes

In https://travis-ci.org/bitcoin/bitcoin/jobs/420091638:

Pull Request #14036 travis: Compile with sanitizers and clang in one of the linux jobs

doesn't seem to be using clang at all.

Yes, it is not possible to set the compiler when building from depends. I will add this back, when it is possible.

utACK fad8afa3883675880dfa7fa6664fe658f10b11b5