llvm-libc is expected to be built with sanitizers and not use interceptors in
the long run. For now though, we have a hybrid process, where functions
implemented in llvm-libc are instrumented, and glibc fills and sanitizer
interceptors fill in the rest.
Current sanitizers have an invariant that the REAL(...) function called from
inside of an interceptor is uninstrumented. A lot of interceptors call strlen()
in order to figure out the size of the region to check/poison. Switch these
callsites over to the internal, unsanitized implementation.
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