[DebugInfo] Hold generic MCExpr in AddrPool

This changes the AddrPool infrastructure to enable it to hold
generic MCExpr expressions, not just MCSymbolRefExpr.

This is in preparation for supporting debug info for TLS variables
on PowerPC, where we need to describe the variable location using
a more complex expression than just MCSymbolRefExpr.
[DebugInfo] Allow getDebugThreadLocalSymbol to return MCExpr

This allows getDebugThreadLocalSymbol to return a generic MCExpr
instead of just a MCSymbolRefExpr.

This is in preparation for supporting debug info for TLS variables
on PowerPC, where we need to describe the variable location using
a more complex expression than just MCSymbolRefExpr.
[PowerPC] Support TLS variables in debug info

This adds an implementation of getDebugThreadLocalSymbol for
(64-bit) PowerPC.  This needs to return a generic MCExpr
since on ppc64, we need to add a bias of 0x8000 to the
value returned by the R_PPC64_DTPREL64 relocation.