Summary:
If `toAppend(std::string*, const Object&)` is defined for a particular object
type, make `objectToString()` simply return the exact output of `toAppend()`.

Previously the code returned `(type_name: toAppendResult)`

The new logic is based on the assumption that if `toAppend()` is implemented
for an object, it should emit all relevant information about the object that
may be needed by a developer.

The old behavior of always including type information was potentially helpful
in cases where `objectToString()` was called only in exceptional cases.
(e.g., after an exception thrown by `folly::format()`).  However it made
`objectToString()` too verbose in other non-exceptional situations where we
want to format objects (e.g., in `XCHECK_EQ()`).  There doesn't really seem to
be a strong need to keep the type information in the `folly::format()`
exception case, so this just drops it rather than adding two separate versions
of this function.

Reviewed By: chadaustin

Differential Revision: D14545303

fbshipit-source-id: 29e2f12d65fd22112486ed11f989b63875980088