[libFuzzer][Windows] Use dllexport for all declarations in FuzzerInterface.h
Use dllexport for all declarations in FuzzerInterface.h Use it for clang
even though clang supports default visibility attribute to prevent a
warning from being thrown when LLVMFuzzerMutate is defined with dllexport.
This makes FUZZER_INTERFACE_VISIBILITY (FuzzerInterface.h) consistent with
ATTRIBUTE_INTERFACE (FuzzerDefs.h) when using clang on Windows.
Reviewers: vitalybuka, morehouse
Reviewed By: morehouse
Differential Revision: https://reviews.llvm.org/D57305