[Wasm] weak symbol issue

[yamt]

when you have weak and non-weak references to a non exiting symbol, linker seems to treat them as they were all weak references.
a small demo code: https://github.com/yamt/garbage/tree/master/c/weak

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

[sbc100]

Fix in flight: https://reviews.llvm.org/D144747

[llvmbot]

@llvm/issue-subscribers-lld-wasm