Make Sema::getPrintingPolicy less ridiculously expensive. This used to perform

an identifier table lookup, *and* copy the LangOptions (including various
std::vector<std::string>s). Twice. We call this function once each time we start
parsing a declaration specifier sequence, and once for each call to Sema::Diag.

This reduces the compile time for a sample .c file from the linux kernel by 20%.

llvm-svn: 270009