[C++] Fix ambiguous ANTLRInputStream constructor overload

[jcking]

Not sure when this was added, but std::string is implicitly convertible to std::string_view. So this can cause compilation errors as the compiler does not know which one to select.

[jcking]

@parrt Compilation fix. std::string is implicitly convertible to std::string_view. So the current code causes the compiler to be confused when using something like a string literal.