Clang incorrectly thinks that enumerators of enum does not fit in the largest integer type

[yeah-boi]

Bugzilla Link	24293
Version	trunk
OS	All
CC	@DougGregor

Extended Description

Consider the following well-formed program (prog.cc):

#include
enum E { x = std::numeric_limits::max(), y };
int main() {}

Compile it with the following command line:

clang++ prog.cc -stdlib=libc++ -std=c++14 -pedantic-errors

The enumerators x and y fits in unsigned long long int, but clang incorrectly gives the following error message:

prog.cc:2:57: error: incremented enumerator value 9223372036854775808 is not representable in the largest integer type [-Werror,-Wenum-too-large]
enum E { x = std::numeric_limits::max(), y };
^
1 error generated.