[cmake] Update experimental target error message

Summary:
Update this error message indicate this test only ensures experimental
targets were passed via LLVM_EXPERIMENTAL_TARGETS_TO_BUILD.

Originally, this test validated all targets, but in r184923, it was moved
after the LLVMBUILDTOOL test, which also validates all targets, making
that part of the test redundant.

Differential Revision: https://reviews.llvm.org/D41273

llvm-svn: 321012