[Sema] Classify conversions from enum to float as narrowing

Summary:
According to [dcl.init.list]p7:
  A narrowing conversion is an implicit conversion
  - ...
  - from an integer type or unscoped enumeration type to a
    floating-point type, except where the source is a constant
    expression and the actual value after conversion will fit into
    the target type and will produce the original value when
    converted back to the original type, or
  - ...

Currently clang does not handle the 'unscoped enumeration' case. This
patch fixes the corresponding check.

Reviewers: faisalv, rsmith, rogfer01

Reviewed By: rogfer01

Subscribers: rogfer01, cfe-commits

Differential Revision: https://reviews.llvm.org/D42545

llvm-svn: 325668