Add new static analyzer for checking LLVM coding conventions: -analyzer-check-llvm-conventions

Currently these checks are intended to be largely syntactical, but may get more
sophisticated over time.

As an initial foray into this brave new world, emit a static analyzer warning
when binding a temporary 'std::string' to an 'llvm::StringRef' where the
lifetime of the 'std::string' does not outlive the 'llvm::StringRef'.

llvm-svn: 96147