Fix a narrowing warning due to a type mismatch (size_t vs uint64).

lib/Transforms/Instrumentation/AddressSanitizer.cpp:1405:36: error: non-constant-expression cannot be narrowed from type 'uint64_t' (aka 'unsigned long long') to 'size_t' (aka 'unsigned int') in initializer list [-Wc++11-narrowing]
    getAllocaSizeInBytes(AI),
    ^~~~~~~~~~~~~~~~~~~~~~~~