Add option to disable variant narrowing conversion changes.

The paper P0608R3 - "A sane variant converting constructor" disallows
narrowing conversions in variant. It was meant to address this
surprising problem:

  std::variant<std::string, bool> v = "abc";
  assert(v.index() == 1); // constructs a bool.

However, it also disables every potentially narrowing conversion. For

  variant<unsigned> v = 0; // ill-formed
  variant<string, double> v2 = 42; // ill-formed (int -> double narrows)

These latter changes break code. A lot of code. Within Google it broke
on the order of a hundred thousand target with thousands of root causes
responsible for the breakages.

Of the breakages related to the narrowing restrictions, none of them
exposed outstanding bugs. However, the breakages caused by boolean
conversions (~13 root causes), all but one of them were bugs.

For this reasons, I am adding a flag to disable the narrowing conversion
changes but not the boolean conversions one.

One purpose of this flag is to allow users to opt-out of breaking changes
in variant until the offending code can be cleaned up. For non-trivial
variant usages the amount of cleanup may be significant.

This flag is also required to support automated tooling, such as
clang-tidy, that can automatically fix code broken by this change.
In order for clang-tidy to know the correct alternative to construct,
it must know what alternative was being constructed previously, which
means running it over the old version of std::variant.

Because this change breaks so much code, I will be implementing the
aforementioned clang-tidy check in the very near future.

Additionally I'm plan present this new information to the committee so they can
re-consider if this is a breaking change we want to make.

I think libc++ should very seriously consider pulling this change
before the 9.0 release branch is cut. But that's a separate discussion
that I will start on the lists.

For now this is the minimal first step.

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed