[MC] - Add .stack_size sections into groups and link them with .text

[MC] - Add .stack_size sections into groups and link them with .text

D39788 added a '.stack-size' section containing metadata on function stack sizes
to output ELF files behind the new -stack-size-section flag.

This change does following two things on top:

1) Imagine the case when there are -ffunction-sections flag given and there are text sections in COMDATs.
    The patch adds a '.stack-size' section into corresponding COMDAT group, so that linker will be able to
    eliminate them fast during resolving the COMDATs.
2) Patch sets a SHF_LINK_ORDER flag and links '.stack-size' with the corresponding .text.
   With that linker will be able to do -gc-sections on dead stack sizes sections.

Differential revision: https://reviews.llvm.org/D46874

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335332 91177308-0d34-0410-b5e6-96231b3b80d8