methods are new to Function:

  bool hasCollector() const;
  const std::string &getCollector() const;
  void setCollector(const std::string &);
  void clearCollector();

The assembly representation is as such:

  define void @f() gc "shadow-stack" { ...

The implementation uses an on-the-side table to map Functions to 
collector names, such that there is no overhead. A StringPool is 
further used to unique collector names, which are extremely
likely to be unique per process.

llvm-svn: 44769

llvm-svn: 44710

and the 'pure' parameter attribute to 'readonly'.
Names suggested by DannyB.

llvm-svn: 44273

llvm-svn: 44218

llvm-svn: 44215

drops a dependency on flex and lets us make future progress more 
easily.  Yay for 2 fewer .cvs files to make silly conflicts with.

llvm-svn: 44213