Fix branch name

Author: fzou1

CMakeLists.txt

@@ -84,7 +84,7 @@ if(NOT USE_PREBUILT_LLVM)
         set(CLANG_BASE_REVISION release_11)
     elseif(EXISTS "${LLVM_EXTERNAL_CLANG_SOURCE_DIR}/CMakeLists.txt")
         set(CLANG_SOURCE_DIR "${LLVM_EXTERNAL_CLANG_SOURCE_DIR}")
-        set(CLANG_BASE_REVISION release/10.x)
+        set(CLANG_BASE_REVISION release/11.x)
     endif()
     if(EXISTS ${CLANG_SOURCE_DIR})
         message(STATUS "Using Clang source code direcotry: ${CLANG_SOURCE_DIR}")