other/9274: [pch] cannot create libstdc++-v3 pch

[Geoffrey Keating]

I've confirmed that this problem still exists, at least partly.  On 
Darwin, test1.cc prints:

ld: Undefined symbols:
std::basic_ostream<char, std::char_traits<char> 
 >::operator<<(std::basic_ostream<char, std::char_traits<char> >& 
(*)(std::basic_ostream<char, std::char_traits<char> >&))

and test2.cc prints:

ld: Undefined symbols:
std::basic_string<char, std::char_traits<char>, std::allocator<char> 
 >::basic_string[in-charge](char const*, std::allocator<char> const&)
std::basic_string<char, std::char_traits<char>, std::allocator<char> 
 >::~basic_string [in-charge]()
std::allocator<char>::allocator[in-charge]()
std::allocator<char>::~allocator [in-charge]()
std::basic_ostream<char, std::char_traits<char> >& std::operator<< 
<char, std::char_traits<char>, std::allocator<char> 
 >(std::basic_ostream<char, std::char_traits<char> >&, 
std::basic_string<char, std::char_traits<char>, std::allocator<char> > 
const&)

This is an improvement, but it's not quite working yet.