[libc] Mark operator== const to avoid ambiguity in C++20. (llvm#68805)

C++20 will automatically generate an operator== with reversed operand
order, which is ambiguous with the written operator== when one argument
is marked const and the other isn't.

This operator currently triggers -Wambiguous-reversed-operator at usage
site libc/test/UnitTest/PrintfMatcher.cpp:28.

Author: bazuzi

libc/src/stdio/printf_core/core_structs.h

@@ -53,7 +53,7 @@ struct FormatSection {
 
   // This operator is only used for testing and should be automatically
   // optimized out for release builds.
-  bool operator==(const FormatSection &other) {
+  bool operator==(const FormatSection &other) const {
     if (has_conv != other.has_conv)
       return false;