[libFuzzer] Don't use `#ifdef` for defined macros, instead use `#if`.
[libFuzzer] Don't use `#ifdef` for defined macros, instead use `#if`.

Differential Revision: https://reviews.llvm.org/D28972

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@292670 91177308-0d34-0410-b5e6-96231b3b80d8

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