[clang-fuzzer] Fix incremental builds of the fuzzer

Summary:
Don't use BUILD_IN_SOURCE keep git checkout clean
Don't forward CMAKE_GENERATOR as ExternalProject_Add should do it already
Reset UPDATE_COMMAND to avoid git checkout updates on each build

Reviewers: kcc, morehouse

Subscribers: cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D39445

llvm-svn: 317035