[LibFuzzer] Allow LibFuzzer to be built in modes other than RELEASE.

Previously the flags were only being set correctly when the
build type was "Release". Now the build should work properly
for all the supported build types. When building libFuzzer
the optimization level respects whatever is used for the
rest of LLVM but for the LibFuzzer tests we force -O0.

Differential Revision: http://reviews.llvm.org/D20558

llvm-svn: 270912