Fix BUILD_SHARED_LIBS for clang which broke in D61909
Fix BUILD_SHARED_LIBS for clang which broke in D61909

llvm_add_library ignores `BUILD_SHARED_LIBS` `STATIC` is explicitly specified. This restores the `BUILD_SHARED_LIBS` behavior to the clang build.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@361271 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang