Remove C++14 auto return type on utf8_to_utf32
Remove C++14 auto return type on utf8_to_utf32

- Also remove static instance of std::wstring_convert. Access to
  std::wstring_convert<>::{from,to}_bytes() is not guaranteed to be thread
  safe.

ericvergnaud/antlr4,antlr/antlr4,parrt/antlr4,antlr/antlr4,parrt/antlr4,parrt/antlr4,ericvergnaud/antlr4,ericvergnaud/antlr4,antlr/antlr4,parrt/antlr4,ericvergnaud/antlr4,parrt/antlr4,antlr/antlr4,antlr/antlr4,ericvergnaud/antlr4,parrt/antlr4,ericvergnaud/antlr4,ericvergnaud/antlr4,parrt/antlr4,antlr/antlr4,parrt/antlr4,parrt/antlr4,antlr/antlr4,antlr/antlr4,ericvergnaud/antlr4,parrt/antlr4,antlr/antlr4,ericvergnaud/antlr4,antlr/antlr4,ericvergnaud/antlr4