[clang-repl][NFC] Split weak symbol test to a new test

Windows has some issues when we try to use `__attribute__((weak))` in
JIT, so we disabled that. But it's not worth to disable the whole test
just for this single feature. This patch split that part from the
original test so we can keep testing stuff that normally working in
Windows.

Signed-off-by: Jun Zhang <jun@junz.org>

Differential Revision: https://reviews.llvm.org/D129250