swift234

[PGO] Add SILMetadata for branch weights.

SILMetadata is the base class with a single enum member (MDKind).
SILBranchNode is the derived class with additional members:
  unsigned NumOperands
  an array of uint32_t

A static member function SILBranchNode::get is implemented to get or create
SILBranchNode. All SILMetadata created are uniqued and saved in SILModule's
member variable:
  llvm::FoldingSet<SILMetadata> Metadatas

Usage of SILMetadta by SILInstruction is captured in SILModule's member variable:
  llvm::DenseMap<const SILInstruction *, SILMetadata *> MetadataStore
This is similar to LLVM's Metadata. Another option is to add a SILMetadata* to
SILInstruction. The disadvantage is the waste of space when we don't have PGO on.

This commit also enables parsing and printing of SILMetadata.

We add keyword sil_metadata to define SILMetadata:
  sil_metadata !0 = {"branch_weights", 3, 5}

For parsing, we add a map in SILModule
  llvm::DenseMap<unsigned, SILMetadata *> NumberedMetadata
that maps from ID to SILMetadata* to help matching usage of "!id" in SILFunction
with definition of "!id" in sil_metadata section.

For printing, we assign IDs to SILMetadata at SILModule scope, we then pass in
an optional argument of
  llvm::DenseMap<const SILMetadata *, unsigned> *MetadataMap
to SILFunction::print in order to get the ID of SILMetadata used in
SILInstruction.

Post-commit review will be appreciated.

rdar://18269754


Swift SVN r23713