-fno-semantic-interposition: Don't set dso_local on GlobalVariable

`clang -fpic -fno-semantic-interposition` may set dso_local on variables for -fpic.

GCC folks consider there are 'address interposition' and 'semantic interposition',
and 'disabling semantic interposition' can optimize function calls but
cannot change variable references to use local aliases
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100483).

This patch removes dso_local for variables in
`clang -fpic -fno-semantic-interposition` mode so that the built shared objects can
work with copy relocations. Building llvm-project tiself with
-fno-semantic-interposition (D102453) should now be safe with trunk Clang.

Example:
```
// a.c
int var;
int *addr() { return var; }

// old: cannot be interposed
movslq  .Lvar$local(%rip), %rax
// new: can be interposed
movq    var@GOTPCREL(%rip), %rax
movslq  (%rax), %rax
```

The local alias lowering for `GlobalVariable`s is kept in case there is a
future option allowing local aliases.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D102583