[gn build] Make `ninja check-clang` also run Clang's unit tests

Also add a build file for clang/lib/ASTMatchers/Dynamic, which is only needed
by tests (and clang/tools/extra).

Also make llvm/utils/gn/build/sync_source_lists_from_cmake.py check that every
CMakeLists.txt file below {lld,clang}/unittests has a corresponding BUILD.gn
file, so we notice if new test binaries get added (since the failure mode for
missing GN build files for tests is just the tests silently not running in the
GN build).

Also add a unittest() macro for defining unit test targets, and add a lengthy
comment there about where the unit test binaries go and why.

With this, the build files for //clang are complete.

Differential Revision: https://reviews.llvm.org/D56116

llvm-svn: 350171