Provide a hook for the benefit of clients using clang IR gen as a subroutine:

emit metadata associating allocas and global values with a Decl*.  This feature
is controlled by an option that (intentionally) cannot be enabled on the command
line.

To use this feature, simply set
  CodeGenOptions.EmitDeclMetadata = true;
and then interpret the completely underspecified metadata. :)

llvm-svn: 107739