[lldb] StructuredData should not truncate uint64_t values

In json::Value, getAsInteger returns an optional<int64_t> and getAsNumber
returns an optional<double>. If a value is larger than what an int64_t
can hold but smaller than what a uint64_t can hold, the getAsInteger
function will fail but the getAsNumber will succeed. However, the value
shouldn't be interpreted as a double.

rdar://105556974

Differential Revision: https://reviews.llvm.org/D144238