[libFuzzer] Remove optimization flags for tests.

We need to build all the tests with -O0, otherwise optimizations may merge some
basic blocks and the tests will fail.
In this diff, I simplify the cmake implementation and I remove the flags for
Windows too (/O[123s]).

Differential Revision: https://reviews.llvm.org/D28632

llvm-svn: 292737
1 file changed