[scudo] Add Last entry to ReleaseToOS enum. (#188645)

This allows static asserts to be set in tracing code that might use the
ReleaseToOS values as indexes.

This would have caused a compile failure instead of a runtime crash when
I added the use of a new ReleaseToOS value.

compiler-rt/lib/scudo/standalone/common.h

@@ -248,6 +248,7 @@ enum class ReleaseToOS : u8 {
              // take.
   ForceFast, // Force release pages to the OS, but do it quickly and skip any
              // cases where a lock is held by another thread.
+  Last = ForceFast, // Must be set to the last entry in the enum.
 };
 
 constexpr unsigned char PatternFillByte = 0xAB;