[MC][ELF] Improve st_size propagation rule

`.symver foo, foo@ver` creates the MCSymbolELF `foo@ver` whose almost all
attributes (including st_size) should inherit from `foo` (GNU as behavior).

a041ef1b added st_size propagation which works
for many cases but fails for the following one:

```
.set __GLIBC_2_12_sys_errlist, _sys_errlist_internal
.type   __GLIBC_2_12_sys_errlist,@object
.size   __GLIBC_2_12_sys_errlist, 1080
.symver __GLIBC_2_12_sys_errlist, sys_errlist@GLIBC_2.12
...
_sys_errlist_internal:
.size   _sys_errlist_internal, 1072
```

`sys_errlist@GLIBC_2.12`'s st_size is 1072 (incorrect), which does not match
`__GLIBC_2_12_sys_errlist`'s st_size: 1080.

The problem is that `Base` is (the final) `_sys_errlist_internal` while we want
to respect (the intermediate) `__GLIBC_2_12_sys_errlist`'s st_size.
Fix this by following the MCSymbolRefExpr assignment chain and finding
the closest non-null `getSize()`, which covers most needs. Notably MCBinaryExpr
is not handled, but it is rare enough to matter.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D123283