[c++20] Add CXXRewrittenBinaryOperator to represent a comparison

operator that is rewritten as a call to multiple other operators.

No functionality change yet: nothing creates these expressions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375305 91177308-0d34-0410-b5e6-96231b3b80d8