Fix the build when LLVM_TARGETS_TO_BUILD is empty.

Using LLVMTestingSupport in the LLVM_LINK_COMPONENTS breaks the build when
LLVM_TARGETS_TO_BUILD is set to empty.

Libraries that depend on LLVMTestingSupport need to use
target_link_libraries(<target> PRIVATE LLVMTestingSupport) instead.

This required change was already commited by r341899 to fix another build
issue.

This fixes rdar://problem/44615064.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342593 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed