We use LLVM_BUILD_LLVM_DYLIB to build the llvm libraries as shared, but that
does not work for clang. We can use BUILD_SHARED_LIBS on clang, but since we
build them all together, we need to smuggle it in using this patch.

Index: CMakeLists.txt
===================================================================
--- tools/clang/CMakeLists.txt
+++ tools/clang/CMakeLists.txt
@@ -1,5 +1,10 @@
 cmake_minimum_required(VERSION 3.4.3)
 
+if( CLANG_BUILD_SHARED_LIBS )
+  set(BUILD_SHARED_LIBS ON)
+  set(LLVM_BUILD_LLVM_DYLIB OFF)
+endif()
+
 # If we are not building as a part of LLVM, build Clang as an
 # standalone project, using LLVM as an external library:
 if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )