[CMake] Replicate what ClangConfig does for unified builds (#70890)

ClangConfig sets `CLANG_INCLUDE_DIRS` and Swift build system uses that
variable in a couple of places. Without this, several of the include
directories for Clang might not be added to different parts of the
compiler, like when using SwiftSourceSupport files. Before these
changes, headers like `clang/CAS/CASOptions.h` were not being found.

Author: drodriguez

cmake/modules/SwiftSharedCMakeConfig.cmake

@@ -272,9 +272,11 @@ macro(swift_common_unified_build_config product)
   set(${product}_NATIVE_CLANG_TOOLS_PATH "${CMAKE_BINARY_DIR}/bin")
   set(LLVM_PACKAGE_VERSION ${PACKAGE_VERSION})
   set(LLVM_CMAKE_DIR "${CMAKE_SOURCE_DIR}/cmake/modules")
+  set(CLANG_INCLUDE_DIRS
+    "${LLVM_EXTERNAL_CLANG_SOURCE_DIR}/include"
+    "${CMAKE_BINARY_DIR}/tools/clang/include")
 
-  include_directories(${LLVM_EXTERNAL_CLANG_SOURCE_DIR}/include
-    ${CMAKE_BINARY_DIR}/tools/clang/include)
+  include_directories(${CLANG_INCLUDE_DIRS})
 
   # If cmark was checked out into tools/cmark, expect to build it as
   # part of the unified build.