[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

llvm-svn: 298424