Previously, overflow and underflow both caused this to return `nil`, which causes several problems:
* It does not distinguish between a large but valid input and a malformed input. `Float("3.402824e+38")` is perfectly well-formed but returns nil
* It differs from how the compiler handles literals. As a result, `Float(3.402824e+38)` is very different from `Float("3.402824e+38")`
* It's inconsistent with Foundation Scanner()
* It's inconsistent with other programming languages
This is exactly the same as #25313
Fixes rdar://problem/36990878
This adjusts the standard library test suite to mostly pass on Windows.
The remaining failures are due to various cases:
- memory corruption (`_swift_stdlib_free` in swiftDemangle)
- heap corruption (canGrowUsingRealloc)
- withVAList failure (unresolved)
- unicode handling on the command line
