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

include/llvm/IR/Module.h

@@ -502,6 +502,8 @@ public:
   /// Materializer.
   std::error_code materializeAllPermanently();
 
+  std::error_code materializeMetadata();
+
 /// @}
 /// @name Direct access to the globals list, functions list, and symbol table
 /// @{