[ASan] Always build shared ASan runtime on Linux.

This commit changes the strategy for building shared ASan runtime
and the way we test it:
  - COMPILER_RT_BUILD_SHARED_ASAN CMake option is removed. We now
    always build shared ASan runtime (it is the default on Android,
    Windows and Mac, and not the default on Linux and FreeBSD).
  - Platforms, which use static runtime by default now have
    "check-asan-dynamic" testsuite. This testsuite contains instrumented
    unit tests, and ASan lit tests, and runs them with shared ASan
    runtime. This testsuite is *not* a part of "check-asan" and
    *not* a part of "check-all", as adding 1000 more test cases, which
    duplicate existing ones is costly. However, you're welcome to
    add this command to your buildbot.

llvm-svn: 224470

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