When computing the composite pointer type for relational comparisons,

When computing the composite pointer type for relational comparisons,
equality comparisons, and conditional operators, produce a composite
pointer type with the appropriate additional "const" qualifiers if the
pointer types would otherwise be incompatible. This is a small
extension (also present in GCC and EDG in a slightly different form)
that permits code like:

  void** i; void const** j;
  i == j;

with the following extwarn:

t.cpp:5:5: warning: comparison of distinct pointer types ('void **' and
      'void const **') uses non-standard composite pointer type
      'void const *const *' [-pedantic]
  i == j;
  ~ ^  ~

Fixes PR6346, and I'll be filing a core issue about this with the C++
committee.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@97177 91177308-0d34-0410-b5e6-96231b3b80d8