fix MSVC C5219 in to_ascii
fix MSVC C5219 in to_ascii

Summary:
Fixes the following warning:

```
folly/lang/ToAscii.h(224): warning C5219: implicit conversion from 'T' to 'double', possible loss of data
        with
        [
            T=size_t
        ]
folly/lang/ToAscii.h(236): note: see reference to function template instantiation 'size_t folly::detail::to_ascii_size_clzll<10>(uint64_t)' being compiled
folly/lang/ToAscii.h(349): note: see reference to function template instantiation 'size_t folly::detail::to_ascii_size_route<10>(uint64_t)' being compiled
folly/lang/ToAscii.h(358): note: see reference to function template instantiation 'size_t folly::to_ascii_size<10>(uint64_t)' being compiled
```

Reviewed By: luciang

Differential Revision: D31314625

fbshipit-source-id: b32a566b67d2f04225b59ee682931d5f8d9ede1e

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly