Significantly change the way we build ASan unittests in CMake

Significantly change the way we build ASan unittests in CMake
build tree. Now just-built Clang is used to:
  1) compile instrumented sources (as before);
  2) compile non-instrumented sources;
  3) compile our own instrumented version of googletest;
  4) link it all together using -fsanitize=address flag
     (instead of trying to copy linker behavior in
      CMake build rules).

This makes ASan unittests pretty much self-consistent
and independent of other LLVM libraries.

llvm-svn: 170541