Add bool to DebugLocDwarfExpression to control emitting comments.

DebugLocDwarfExpression::EmitOp was creating temporary strings by concatenating Twine's.

When emitting to object files, these comments are thrown away.

This commit adds a boolean to the constructor of the DwarfExpression to control whether it will actually emit
any comments.  This prevents it from even generating the temporary comments which would have been thrown away anyway.

llvm-svn: 237827