Adding a collector name attribute to Function in the IR. These

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