Improve Format's handling of temporaries

Improve Format's handling of temporaries

Summary:
`FormatValue` holds non-integral objects by reference, which
can cause subtle problems if a formatter is constructed with temporary
arguments and used beyond the expression that constructs it.

With this diff the arguments are perfectly forwarded into a tuple, so
the formatter will take ownership of the temporaries while holding
references to lvalues as before.

The only downside is that now `FormatValue` objects are constructed
every time the formatter is used, rather than only at formatter
construction. This should not be noticeable as those objects'
constructors should just take a reference to the argument.

Note that the format string is still held by reference, but this is
fine because it should almost always be a string literal.

Reviewed By: ericniebler

Differential Revision: D5317382

fbshipit-source-id: ef8355194b634d3751ef1ccca32dd1db29e27c48