[dotest] Make the set of tests independent of the test configuration

Summary:
In the magic test duplicator, we were making the decision whether to
create a test variant based on the compiler and the target platform.
This meant that the set of known tests was different for each test
configuration.

This patch makes the set of generated test variants static and handles
the skipping via runtime checks instead. This is more consistent with
how we do other test-skipping decision (e.g. for libc++ tests), and
makes it easier to expose the full set of tests to lit, which now does
not need to know anything about what things can potentially cause tests
to appear or disappear.

Reviewers: JDevlieghere, aprantl

Subscribers: eraman, lldb-commits

Differential Revision: https://reviews.llvm.org/D45949

llvm-svn: 330708