Built-in equality and relational operators have return type "bool" in C++,

Built-in equality and relational operators have return type "bool" in C++,
not "int".

Fix a typo in the promotion of enumeration types that was causing some
integral promotions to look like integral conversions (leading to
extra ambiguities in overload resolution).

Check for "acceptable" overloaded operators based on the types of the
arguments. This is a somewhat odd check that is specified by the
standard, but I can't see why it actually matters: the overload
candidates it suppresses don't seem like they would ever be picked as
the best candidates.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59583 91177308-0d34-0410-b5e6-96231b3b80d8