[CMake] Quick-Fix targets don't exist when building against LLVM install-tree...

[CMake] Quick-Fix targets don't exist when building against LLVM install-tree with LLDB_INCLUDE_TESTS=ON

The issue came up during release testing for LLVM 8: https://bugs.llvm.org/show_bug.cgi?id=40443

Differential Revision: https://reviews.llvm.org/D57233

llvm-svn: 352382