Enhance the diagnostic for literal float -> int conversions to suggest

Enhance the diagnostic for literal float -> int conversions to suggest
rewriting the literal when the value is integral. It is not uncommon to
see code written as:

  const int kBigNumber = 42e5;

Without any real awareness that this is no longer an ICE. The note helps
automate and ease the process of fixing code that violates the warning.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@129243 91177308-0d34-0410-b5e6-96231b3b80d8