[libFuzzer] Disable implicit builtin knowledge about memcmp-like functions...

[libFuzzer] Disable implicit builtin knowledge about memcmp-like functions when -fsanitize=fuzzer-no-link is given.

Summary: This patch disables implicit builtin knowledge about memcmp-like functions when compiling the program for fuzzing, i.e., when -fsanitize=fuzzer(-no-link) is given. This allows libFuzzer to always intercept memcmp-like functions as it effectively disables optimizing calls to such functions into different forms. This is done by adding a set of flags (-fno-builtin-memcmp and others) in the clang driver. Individual -fno-builtin-* flags previously used in several libFuzzer tests are now removed, as it is now done automatically in the clang driver.

Reviewers: morehouse, hctim

Subscribers: cfe-commits, #sanitizers

Tags: #clang, #sanitizers

Differential Revision: https://reviews.llvm.org/D83987