Added FOLLY_IGNORE and FOLLY_SEMICOLON helper macros

Summary: these macros are generally useful for other higher-order macros

Reviewed By: yfeldblum

Differential Revision: D3194777

fb-gh-sync-id: 667fc51c681786ad422309ee463881dc22c972f7
fbshipit-source-id: 667fc51c681786ad422309ee463881dc22c972f7

folly/Preprocessor.h

@@ -72,6 +72,16 @@
  */
 #define FB_SINGLE_ARG(...) __VA_ARGS__
 
+/**
+ * Helper macro that just ignores its parameters.
+ */
+#define FOLLY_IGNORE(...)
+
+/**
+ * Helper macro that just ignores its parameters and inserts a semicolon.
+ */
+#define FOLLY_SEMICOLON(...) ;
+
 /**
  * FB_ANONYMOUS_VARIABLE(str) introduces an identifier starting with
  * str and ending with a number that varies with the line.