Bug 1791961 - accessible: Fix C++20 -Wambiguous-reversed-operator warnings. r=Jamie

clang is warning that C++20 expects comparison operators to be commutative: `a == b` and `b == a` should resolve to the same comparison operator function. Warnings about the comparison of const and non-const objects can be fixed by making the comparison operator function const.

accessible/base/TextAttrs.h:139:54 [-Wambiguous-reversed-operator] ISO C++20 considers use of overloaded operator '!=' (with operand types 'mozilla::a11y::TextAttrsMgr::TextDecorValue' and 'mozilla::a11y::TextAttrsMgr::TextDecorValue') to be ambiguous despite there being a unique best viable function with non-reversed arguments
...

Differential Revision: https://phabricator.services.mozilla.com/D179019

accessible/base/TextAttrs.h

@@ -378,11 +378,13 @@ class TextAttrsMgr {
       return bool(mLine & mozilla::StyleTextDecorationLine::LINE_THROUGH);
     }
 
-    bool operator==(const TextDecorValue& aValue) {
+    bool operator==(const TextDecorValue& aValue) const {
       return mColor == aValue.mColor && mLine == aValue.mLine &&
              mStyle == aValue.mStyle;
     }
-    bool operator!=(const TextDecorValue& aValue) { return !(*this == aValue); }
+    bool operator!=(const TextDecorValue& aValue) const {
+      return !(*this == aValue);
+    }
 
    private:
     nscolor mColor;