Improve handling of enumerator values for C and C++, including:

  - In C++, prior to the closing '}', set the type of enumerators
    based on the type of their initializer. Don't perform unary
    conversions on the enumerator values.
  - In C++, handle overflow when an enumerator has no initializer and
    its value cannot be represented in the type of the previous
    enumerator.
  - In C, handle overflow more gracefully, by complaining and then
    falling back to the C++ rules.
  - In C, if the enumerator value is representable in an int, convert the
    expression to the type 'int'.

Fixes PR5854 and PR4515.

llvm-svn: 95031