[WebAssembly] Honor --allow-undefined even for explicit exports

When a symbol is exported via --export=foo but --allow-undefined
is also specified, the symbol is now allowed to be undefined.
Previously we were special casing such symbols.

This combinations of behavior is exactly what emescripten
requires. Although we are trying hard not to allow emscripten
specific features in lld, this one makes sense.

Enforce this behavior by added this case to test/wasm/undefined.ll.

Differential Revision: https://reviews.llvm.org/D44237

llvm-svn: 326976