Port libFuzzer tests to LIT. Do not require two-stage build for check-fuzzer.

This revision ports all libFuzzer tests apart from the unittest to LIT.
The advantages of doing so include:

 - Tests being self-contained
 - Much easier debugging of a single test
 - No need for using a two-stage compilation

The unit-test is still compiled using CMake, but it does not need a
freshly built compiler.

NOTE: The previous two-stage bot configuration will NOT work, as in the
second stage build LLVM_USE_SANITIZER is set, which disables ASAN from
being built.
Thus bots will be reconfigured in the next few commits.

Differential Revision: https://reviews.llvm.org/D36295

llvm-svn: 310075