[CMake] Introduce "llvm_add_library(SHARED STATIC)" to build both shared lib...

[CMake] Introduce "llvm_add_library(SHARED STATIC)" to build both shared lib and static lib simulataneously.

  llvm_add_library(foo SHARED STATIC
    DEPENDS <dependent targets...>
    LINK_LIBS <required libraries...>
    )

It generates both foo (foo.so) and foo_static(foo.a) and both of them depend on DEPENDS and LINK_LIBS.
Then, also obj.foo is generated. obj.foo depends on DEPENDS, but doesn't depend on LINK_LIBS.

llvm-svn: 201854