[ubsan] Re-commit: lit changes for lld testing, future lto testing.

Summary:
As discussed in https://github.com/google/oss-fuzz/issues/933,
it would be really awesome to be able to use ThinLTO for fuzzing.
However, as @kcc has pointed out, it is currently undefined (untested)
whether the sanitizers actually function properly with LLD and/or LTO.

This patch is inspired by the cfi test, which already do test with LTO
(and/or LLD), since LTO is required for CFI to function.

I started with UBSan, because it's cmakelists / lit.* files appeared
to be the cleanest. This patch adds the infrastructure to easily add
LLD and/or LTO sub-variants of the existing lit test configurations.

Also, this patch adds the LLD flavor, that explicitly does use LLD to link.
The check-ubsan does pass on my machine. And to minimize the [initial]
potential buildbot breakage i have put some restrictions on this flavour.

Please review carefully, i have not worked with lit/sanitizer tests before.

The original attempt, r319525 was reverted in r319526 due
to the failures in compiler-rt standalone builds.

Reviewers: eugenis, vitalybuka

Reviewed By: eugenis

Subscribers: #sanitizers, pcc, kubamracek, mgorny, llvm-commits, mehdi_amini, inglorion, kcc

Differential Revision: https://reviews.llvm.org/D39508

llvm-svn: 319575

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