disable conversion warnings in folly/Format

Summary: as title. Disabling -Wconversion since it can be triggered pretty easily. We've run into this a lot in cachelib code. Until now, we've manually disabled -Wconversion around the header for folly/Format.h in our own code. But I think it's much easier just to disable it inside Format-inl.h itself.

Reviewed By: yfeldblum

Differential Revision: D14471124

fbshipit-source-id: df6bcbd57cbbdb0e6a762eaa5d11548c9e614e1b

folly/Format-inl.h

@@ -32,9 +32,10 @@
 #include <folly/lang/Exception.h>
 #include <folly/portability/Windows.h>
 
-// Ignore -Wformat-nonliteral warnings within this file
+// Ignore -Wformat-nonliteral and -Wconversion warnings within this file
 FOLLY_PUSH_WARNING
 FOLLY_GNU_DISABLE_WARNING("-Wformat-nonliteral")
+FOLLY_GNU_DISABLE_WARNING("-Wconversion")
 
 namespace folly {