[ELF] - Linkerscript: support all kinds of sorting (including nested).

Previously we supported only sorting by name.

When there are nested section sorting commands in linker script, there can be at most 1
level of nesting for section sorting commands.

SORT_BY_NAME (SORT_BY_ALIGNMENT (wildcard section pattern)). It will sort the input
sections by name first, then by alignment if 2 sections have the same name.

SORT_BY_ALIGNMENT (SORT_BY_NAME (wildcard section pattern)). It will sort the input
sections by alignment first, then by name if 2 sections have the same alignment.

SORT_BY_NAME (SORT_BY_NAME (wildcard section pattern)) is treated the same as SORT_
BY_NAME (wildcard section pattern).

SORT_BY_ALIGNMENT (SORT_BY_ALIGNMENT (wildcard section pattern)) is treated the
same as SORT_BY_ALIGNMENT (wildcard section pattern).

All other nested section sorting commands are invalid.

Patch implements that all above.

Differential revision: https://reviews.llvm.org/D23019

llvm-svn: 277583