LLDB standalone build: check if Clang was built independently from LLVM

This is fix for bug 23704: LLDB standalone build always include
ClangConfig.cmake even if Clang was built with LLVM (ClangConfig.cmake
doesn't exist).

Patch by: Eugene Zelenko


git-svn-id: https://llvm.org/svn/llvm-project/lldb/branches/release_36@240419 91177308-0d34-0410-b5e6-96231b3b80d8