[HP aCC 3.76] many remarks #4271-D: type conversion may lose sign from <string>

A couple of remarks from string.cc are making the latest HP aCC builds very noisy:

$ gmake _CXXOPTS="+W2193 +W2261 +W2340 +W2401 +W4235 +W4253 +W4263 +W4264 +W4296 +W4272" t
aCC -c -I/amd/devco/sebor/stdcxx/include/ansi -I/usr/include -mt -I/amd/devco/sebor/stdcxx/include -I/build/sebor/aCC-3.76.b6-12D/include -I/amd/devco/sebor/stdcxx/../rwtest/include -I/amd/devco/sebor/stdcxx/tests/include -Aa +nostl +W2193 +W2261 +W2340 +W2401 +W4235 +W4253 +W4263 +W4264 +W4296 +W4272 +O2 +DD64 +w +W392 +W655 +W684 +W818 +W819 +W849 t.cpp
"/amd/devco/sebor/stdcxx/include/string.cc", line 561: remark #4271-D: type
conversion may lose sign
_s.replace (first1 - __s._C_make_iter (_s._C_data),
^
detected during instantiation of "std::basic_string<_CharT, _Traits,
_Allocator> &std::basic_string<_CharT, _Traits,
_Allocator>::replace(_Allocator::pointer,
_Allocator::pointer, _InputIter, _InputIter, void *) [with
_CharT=char, _Traits=std::char_traits<char>,
_Allocator=std::allocator<char>, _InputIter=const char *]"
at line 634 of "/amd/devco/sebor/stdcxx/include/string"

"/amd/devco/sebor/stdcxx/include/string.cc", line 562: remark #4271-D: type
conversion may lose sign
__last1 - __first1,
^
detected during instantiation of "std::basic_string<_CharT, _Traits,
_Allocator> &std::basic_string<_CharT, _Traits,
_Allocator>::replace(_Allocator::pointer,
_Allocator::pointer, _InputIter, _InputIter, void *) [with
_CharT=char, _Traits=std::char_traits<char>,
_Allocator=std::allocator<char>, _InputIter=const char *]"
at line 634 of "/amd/devco/sebor/stdcxx/include/string"

aCC t.o -o t -L/build/sebor/aCC-3.76.b6-12D/rwtest -lrwtest12D -Aa +nostl -Wl,+s -Wl,+vnocompatwarnings -mt +DD64 -L/build/sebor/aCC-3.76.b6-12D/lib -Wl,+b/build/sebor/aCC-3.76.b6-12D/lib:/build/sebor/aCC-3.76.b6-12D/rwtest -lstd12D -lm