CMake: Fix public header search for generating Xcode/MSVC projects.

Previously, we only had support for one level of library under lib/,
with the existence of the two-level lib/StaticAnalyzer/* hardcoded in
the top-level CMakeLists.txt. This became a problem with split of
libRewrite into several libraries -- with the same sub-names as the
libraries in lib/StaticAnalyzer/.

Now, we match up anything under lib/ to the corresponding directory
in include/clang/.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166505 91177308-0d34-0410-b5e6-96231b3b80d8