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 :-)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232221 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Module.cpp

@@ -413,6 +413,12 @@ std::error_code Module::materializeAllPermanently() {
   return std::error_code();
 }
 
+std::error_code Module::materializeMetadata() {
+  if (!Materializer)
+    return std::error_code();
+  return Materializer->materializeMetadata();
+}
+
 //===----------------------------------------------------------------------===//
 // Other module related stuff.
 //