[libFuzzer] Unpoison parameters before calling user callback.

Summary:
Fixes an MSan false positive when compiling with
-fsanitize=memory,fuzzer.

See https://github.com/google/oss-fuzz/issues/2369 for more details.

Reviewers: kcc

Reviewed By: kcc

Subscribers: llvm-commits, metzman, eugenis

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61753

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer@360390 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/FuzzerExtFunctions.def b/FuzzerExtFunctions.def
index 288a59c..41fa0fd 100644
--- a/FuzzerExtFunctions.def
+++ b/FuzzerExtFunctions.def
@@ -46,3 +46,4 @@
 EXT_FUNC(__msan_scoped_disable_interceptor_checks, void, (), false);
 EXT_FUNC(__msan_scoped_enable_interceptor_checks, void, (), false);
 EXT_FUNC(__msan_unpoison, void, (const volatile void *, size_t size), false);
+EXT_FUNC(__msan_unpoison_param, void, (size_t n), false);