[LTO] Add SelectionKind to IRSymtab and use it in ld.lld/LLVMgold

[LTO] Add SelectionKind to IRSymtab and use it in ld.lld/LLVMgold

In PGO, a C++ external linkage function `foo` has a private counter
`__profc_foo` and a private `__profd_foo` in a `comdat nodeduplicate`.

A `__attribute__((weak))` function `foo` has a weak hidden counter `__profc_foo`
and a private `__profd_foo` in a `comdat nodeduplicate`.

In `ld.lld a.o b.o`, say a.o defines an external linkage `foo` and b.o
defines a weak `foo`. Currently we treat `comdat nodeduplicate` as `comdat any`,
ld.lld will incorrectly consider `b.o:__profc_foo` non-prevailing.  In the worst
case when `b.o:__profd_foo` is retained and `b.o:__profc_foo` isn't, there will
be dangling reference causing an `undefined hidden symbol` error.

Add SelectionKind to `Comdat` in IRSymtab and let linkers ignore nodeduplicate comdat.

Differential Revision: https://reviews.llvm.org/D106228