Add dummy CMake target for *.pass.cpp tests when LIBCXX_CONFIGURE_IDE=ON.

In order for IDE's like CLion to correctly parse and highlight the tests
it needs to know roughly how to build them. This patch adds a dummy CMake target
for each/all of the .pass.cpp tests in the test suite to solve this problem.
The target is only created when LIBCXX_CONFIGURE_IDE=ON, so it shouldn't affect
most users.

Originally I wasn't sure that this change deserved to live upstream, but it's
quite frustrating to edit libc++ tests using CLion or Visual Studio without it,
in particular the filesystem tests which rely heavily on macros. Even though the change
should have no effect on non-IDE users/configurations I decided to commit it upstream
with the hopes it will benefit somebody other than me.

llvm-svn: 307118