[DebugInfo]: Allow DwarfCompileUnit to have line table symbol

Previously line table symbol was represented as `DIE::value_iterator`
inside `DwarfCompileUnit` and subsequent function `intStmtList`
was used to create a local `MCSymbol` to initialize it.

This patch removes `DIE::value_iterator` from `DwarfCompileUnit`
and intoduce `MCSymbol` for representing this units symbol for
`debug_line` section. As a result `applyStmtList` is also modified
 to utilize this. Further more a helper function `getLineTableStartSym`
is also introduced to get this symbol, this would be used by clients
which need to access this line table, i.e `debug_macro`.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D77489