lib/AST/CMakeLists.txt: Update LLVM_USED_LIBS, not to miss symbols with building shared lib. Thanks to Ryuta Suzuki.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135864 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/CMakeLists.txt b/lib/AST/CMakeLists.txt
index 63583a0..945b8a6 100644
--- a/lib/AST/CMakeLists.txt
+++ b/lib/AST/CMakeLists.txt
@@ -1,6 +1,6 @@
 set(LLVM_LINK_COMPONENTS support)
 
-set(LLVM_USED_LIBS clangBasic)
+set(LLVM_USED_LIBS clangBasic clangLex)
 
 add_clang_library(clangAST
   APValue.cpp