IR: Use SmallVector instead of std::vector of TrackingMDRef

Don't use std::vector<TrackingMDRef>, since (at least in some versions
of libc++) std::vector apparently copies values on grow operations
instead of moving them.  Found this when I was temporarily deleting the
copy constructor for TrackingMDRef to investigate a performance
bottleneck.

llvm-svn: 266909

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -107,7 +107,12 @@ class BitcodeReaderMetadataList {
   bool AnyFwdRefs;
   unsigned MinFwdRef;
   unsigned MaxFwdRef;
-  std::vector<TrackingMDRef> MetadataPtrs;
+
+  /// Array of metadata references.
+  ///
+  /// Don't use std::vector here.  Some versions of libc++ copy (instead of
+  /// move) on resize, and TrackingMDRef is very expensive to copy.
+  SmallVector<TrackingMDRef, 1> MetadataPtrs;
 
   LLVMContext &Context;
 public: