[libFuzzer] Use clang as linker on Windows, to properly include sanitizer libraries.

In order to use sanitizers on Windows, we need to link against many runtime
libraries which will depend on the target being created (executable or dll) and
the c runtime library used (MT/MD).
By default, cmake uses link.exe for linking, which fails because we don't
specify the appropiate dependencies. As we don't want to consider all of that
possible situations which depends on the implementation of the compiler-rt, the
simplest option is to change the rules for linking executables and shared
libraries, using the compiler instead of link.exe.
Clang driver will consider the sanitizer flags, and automatically provide the
required libraries to the linker.

Differential Revision: https://reviews.llvm.org/D27869

llvm-svn: 292669