Treat the range of representable values of floating-point types as [-inf, +inf] not as [-max, +max].

Summary:
Prior to r329065, we used [-max, max] as the range of representable
values because LLVM's `fptrunc` did not guarantee defined behavior when
truncating from a larger floating-point type to a smaller one. Now that
has been fixed, we can make clang follow normal IEEE 754 semantics in this
regard and take the larger range [-inf, +inf] as the range of representable
values.

In practice, this affects two parts of the frontend:
 * the constant evaluator no longer treats floating-point evaluations
   that result in +-inf as being undefined (because they no longer leave
   the range of representable values of the type)
 * UBSan no longer treats conversions to floating-point type that are
   outside the [-max, +max] range as being undefined

In passing, also remove the float-divide-by-zero sanitizer from
-fsanitize=undefined, on the basis that while it's undefined per C++
rules (and we disallow it in constant expressions for that reason), it
is defined by Clang / LLVM / IEEE 754.

Reviewers: rnk, BillyONeal

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63793

llvm-svn: 365272

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