[analyzer] Don't treat lambda-captures float constexprs as undefined

RegionStore has special logic to evaluate captured constexpr variables.
However, if the constexpr initializer cannot be evaluated as an integer, the
value is treated as undefined. This leads to false positives when, for example,
a constexpr float is captured by a lambda.

To fix this, treat a constexpr capture that cannot be evaluated as unknown
rather than undefined.

rdar://problem/35784662

llvm-svn: 319638