Rename CanFitInto64Bits to alwaysFitsInto64Bits per discussion on IRC.

This makes the behavior clearer concerning literals with the maximum
number of digits. For a 32-bit example, 4,000,000,000 is a valid uint32_t,
but 5,000,000,000 is not, so we'd have to count 10-digit decimal numbers
as "unsafe" (meaning we have to check for overflow when parsing them,
just as we would for numbers with 11 digits or higher). This is the same,
only with 64 bits to play with.

No functionality change.

llvm-svn: 164639