[fuzzer] Replace FuzzerExtFunctionsDlsymWin.cpp with FuzzerExtFunctionsWeakAlias.cpp

Summary:
Replace FuzzerExtFunctionsDlsymWin.cpp with FuzzerExtFunctionsWeakAlias.cpp
to get externally defined functions (eg: LLVMFuzzerInitialize,
LLVMFuzzerCustomMutator, etc) working again.

Also enable tests that depended on these functions (on windows)

Reviewers: rnk, morehouse

Reviewed By: rnk, morehouse

Subscribers: rnk, morehouse, mgorny

Differential Revision: https://reviews.llvm.org/D51700

llvm-svn: 342698