Be lazy about loading metadata in IRObjectFile.

This speeds up llvm-ar building lib64/libclangSema.a with debug IR files
from 8.658015807 seconds to just 0.351036519 seconds :-)

llvm-svn: 232221
diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
index 3a88db9..3e8f91f 100644
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -413,6 +413,12 @@
   return std::error_code();
 }
 
+std::error_code Module::materializeMetadata() {
+  if (!Materializer)
+    return std::error_code();
+  return Materializer->materializeMetadata();
+}
+
 //===----------------------------------------------------------------------===//
 // Other module related stuff.
 //