[Dexter] Fix incorrect substitution errors in clang-cl builder

Following 262520a3, tests on windows bots began failing due to an incorrect
substitution in the previous patch, where clang-cl was used instead of
clang_cl.

Also fixes an inconsistency in the builders used for some of the tests
in 'dexter-tests', where %clang++ was used for some tests and %clang for
tests that should have identical RUN lines.