[clang] Allow const variables with weak attribute to be overridden

A variable with `weak` attribute signifies that it can be replaced with
a "strong" symbol link time. Therefore it must not emitted with
"weak_odr" linkage, as that allows the backend to use its value in
optimizations.

The frontend already considers weak const variables as
non-constant (note_constexpr_var_init_weak diagnostic) so this change
makes frontend and backend consistent.

This commit reverses the
  f49573d1 weak globals that are const should get weak_odr linkage.
commit from 2009-08-05 which introduced this behavior. Unfortunately
that commit doesn't provide any details on why the change was made.

This was discussed in
https://discourse.llvm.org/t/weak-attribute-semantics-on-const-variables/62311

Differential Revision: https://reviews.llvm.org/D126324