[libFuzzer] NFC Declare LIBFUZZER_FLAGS_BASE outside of an if-block

The current code relies on the assumption that tests are included only
if LLVM_USE_SANITIZE_COVERAGE is enabled.
This commit makes it easier to relax the assumption in the future, as
the variable LIBFUZZER_FLAGS_BASE is used further in libFuzzer tests.

Differential Revision: https://reviews.llvm.org/D35314

llvm-svn: 307825