[LIT] error if directly named test won't be run indirectly

[LIT] error if directly named test won't be run indirectly

Currently, a LIT test named directly (on the command line) will
be run even if the name of the test file does not meet the rules
to be considered a test in the LIT test configuration files for
its test suite. For example, if the test does not have a
recognised file extension.

This makes it relatively easy to write a LIT test that won't
actually be run. I did in: https://reviews.llvm.org/D82567

This patch adds an error to avoid users doing that. There is a
small performance overhead for this check. A command line option
has been added so that users can opt into the old behaviour.

Differential Revision: https://reviews.llvm.org/D83069