[LibFuzzer] Refactor declaration of tests in CMake.

Add a new CMake function (``add_libfuzzer_test()``) to simplify
declaration of executables for testing LibFuzzer and use it to
reorganise how tests are declared.

Note that configuration of the lit configuration files has been moved
as late as possible because we are going to need to disable some tests
for some platforms and we will need to propagate this information into
the lit configuration.

Note the code for custom mains was removed because no tests are
currently written for this and Kostya seems happy to remove this.

Differential Revision: http://reviews.llvm.org/D20706

llvm-svn: 270958

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