[CMake] fix CLANG_INCLUDE_DIRS CMake export

Summary:
This change should fixes the export of CLANG_INCLUDE_DIRS variable in ClangConfig.cmake.

Unlike for the other variables, CLANG_INSTALL_PREFIX wasn't escaped meaning CLANG_INCLUDE_DIRS
resulting in the path "/include" instead of "${CLANG_INSTALL_PREFIX}/include".

Reviewers: beanz

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D30911

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298424 91177308-0d34-0410-b5e6-96231b3b80d8