analyzer: Add missing 'const' to equiv_class::operator==

analyzer: Add missing 'const' to equiv_class::operator==

This produces a warning in C++20:

/home/test/src/gcc/gcc/analyzer/constraint-manager.cc: In member function ‘bool ana::constraint_manager::operator==(const ana::constraint_manager&) const’:
/home/test/src/gcc/gcc/analyzer/constraint-manager.cc:1610:42: warning: C++20 says that these are ambiguous, even though the second is reversed:
 1610 |     if (!(*ec == *other.m_equiv_classes[i]))
      |                                          ^
/home/test/src/gcc/gcc/analyzer/constraint-manager.cc:1178:1: note: candidate 1: ‘bool ana::equiv_class::operator==(const ana::equiv_class&)’
 1178 | equiv_class::operator== (const equiv_class &other)
      | ^~~~~~~~~~~
/home/test/src/gcc/gcc/analyzer/constraint-manager.cc:1178:1: note: candidate 2: ‘bool ana::equiv_class::operator==(const ana::equiv_class&)’ (reversed)
/home/test/src/gcc/gcc/analyzer/constraint-manager.cc:1178:1: note: try making the operator a ‘const’ member function

gcc/analyzer/ChangeLog:

	* constraint-manager.cc (equiv_class::operator==): Add const
	qualifier.
	* constraint-manager.h (equiv_class::operator==): Likewise.