[libFuzzer][Windows] Use dllexport for all declarations in FuzzerInterface.h

Summary:
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

llvm-svn: 352395