Use Darwin libtool's -no_warning_for_no_symbols if available to silence the "has...

Use Darwin libtool's -no_warning_for_no_symbols if available to silence the "has no symbols" link warning

Building compiler-rt on Darwin produces dozens of meaningless warnings about object files having no symbols during static archive creation. This is very intentional as compiler-rt uses #ifdefs to conditionally compile platform-specific code, and we even have a .cpp source file that only contains static asserts to make sure the environment is configured right. On Linux, this situation is fine and no warning is produced. This patch adds a libtool version detection and if it's new enough, we'll use the -no_warning_for_no_symbols flag that suppresses this warning. Build logs should be much cleaner now!

Differential Revision: https://reviews.llvm.org/D27119

llvm-svn: 288640