Add FOLLY_ASSUME_UNREACHABLE

Summary: Add a macro that informs the compiler that a statement is unreachable, and at the same time suppresses `-Wunreachable-code` warnings that Clang throws up when.

Reviewed By: ohcnivek

Differential Revision: D62024037

fbshipit-source-id: 7c180952e6846a9881940fdb618a5388c467727c

folly/lang/Assume.h

@@ -70,3 +70,13 @@ FOLLY_ALWAYS_INLINE void assume(bool cond) {
 }
 
 } // namespace folly
+
+/**
+ * Inform the compiler that the statement is not reachable at runtime, and
+ * disable compiler warnings.
+ */
+#define FOLLY_ASSUME_UNREACHABLE()                  \
+  FOLLY_PUSH_WARNING                                \
+  FOLLY_CLANG_DISABLE_WARNING("-Wunreachable-code") \
+  ::folly::assume_unreachable();                    \
+  FOLLY_POP_WARNING