[CMake] Support for monorepo layout

Support finding libcxx sources in monorepo style layout.

Differential Revision: https://reviews.llvm.org/D45270

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@329212 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 31af0cb..3a306a6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -332,6 +332,7 @@
   __libcpp_version
   PATHS ${LLVM_MAIN_SRC_DIR}/projects/libcxx/include
         ${LLVM_MAIN_SRC_DIR}/runtimes/libcxx/include
+        ${LLVM_MAIN_SRC_DIR}/../libcxx/include
   NO_DEFAULT_PATH
   NO_CMAKE_FIND_ROOT_PATH
   )