IR: Expose ModuleSlotTracker in Value::print()

Allow callers of `Value::print()` and `Metadata::print()` to pass in a
`ModuleSlotTracker`.  This allows them to pay only once for calculating
module-level slots (such as Metadata).

This is related to PR23865, where there was a huge cost for
`MachineFunction::print()`.  Although I don't have a *particular* user
in mind for this new code, I have hit big slowdowns before when running
`opt -debug`, and I think this will be useful.  Going forward, if
someone hits a big slowdown with `print()` statements, they can create a
`ModuleSlotTracker` and send it through.  Similarly, adding support to
`Value::dump()` and `Metadata::dump()` should be trivial.

I added unit tests to be sure the `print()` functions actually behave
the same way with and without the slot tracker.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240867 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Metadata.h

@@ -124,7 +124,11 @@ public:
   ///
   /// If \c M is provided, metadata nodes will be numbered canonically;
   /// otherwise, pointer addresses are substituted.
+  /// @{
   void print(raw_ostream &OS, const Module *M = nullptr) const;
+  void print(raw_ostream &OS, ModuleSlotTracker &MST,
+             const Module *M = nullptr) const;
+  /// @}
 
   /// \brief Print as operand.
   ///

include/llvm/IR/Value.h

@@ -200,7 +200,10 @@ public:
   void dump() const;
 
   /// \brief Implement operator<< on Value.
+  /// @{
   void print(raw_ostream &O) const;
+  void print(raw_ostream &O, ModuleSlotTracker &MST) const;
+  /// @}
 
   /// \brief Print the name of this Value out to the specified raw_ostream.
   ///