[CMake] Introduce llvm_add_library().

  - MODULE;SHARED;STATIC
      STATIC by default w/o BUILD_SHARED_LIBS.
      SHARED by default w/  BUILD_SHARED_LIBS.
  - OUTPUT_NAME name
      Corresponds to OUTPUT_NAME in target properties.
  - DEPENDS targets...
      Same semantics as add_dependencies().
  - LINK_COMPONENTS components...
      Same as the variable LLVM_LINK_COMPONENTS.
  - LINK_LIBS lib_targets...
      Same semantics as target_link_libraries().
  - ADDITIONAL_HEADERS (implemented in LLVMProcessSources)
      May specify header files for IDE generators.

I suggest llvm_add_library() may be used for inter-project add_library stuff
and also suggest add_***_library() may be used project-specific.

Please be patient that llvm_add_library might be ambiguous against add_llvm_library.
[CMake] add_clang_library(): Use llvm_add_library.

CMAKE_MODULE_LINKER_FLAGS can be removed since llvm_add_library(MODULE) adds same flags to MODULE.