[libfuzzer] Use cl driver mode for tests and enable another test on Windows

[libfuzzer] Use cl driver mode for tests and enable another test on Windows

Summary:
When targeting MSVC: compile using clang's cl driver mode (this is needed for
libfuzzer's exit_on_src_pos feature). Don't use -lstdc++ when linking,
it isn't needed and causes a warning.
On Windows: Fix exit_on_src_pos.test by making sure debug info isn't
overwritten during compilation of second binary by using .exe extension.

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: aprantl, JDevlieghere

Differential Revision: https://reviews.llvm.org/D51757

llvm-svn: 341622