[Chrono] Add missing include <ratio>.

The file uses std::ratio without including the correct header.
Previously ratio was indirectly provided through chrono in libc++ but
that's not the case any longer.

This should fix a build failure with modules enabled:
https://green.lab.llvm.org/green/job/clang-stage2-Rthinlto/5185/console

llvm/include/llvm/Support/Chrono.h

@@ -14,6 +14,7 @@
 
 #include <chrono>
 #include <ctime>
+#include <ratio>
 
 namespace llvm {