[ADT] Add llvm::to_float

Summary:
The function matches the interface of llvm::to_integer, but as we are
calling out to a C library function, I let it take a Twine argument, so
we can avoid a string copy at least in some cases.

I add a test and replace a couple of existing uses of strtod with this
function.

Reviewers: zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34518

llvm-svn: 306096