[WebAssembly] Support weak defined symbols

Model weakly defined symbols as symbols that are both
exports and imported and marked as weak. Local references
to the symbols refer to the import but the linker can
resolve this to the weak export if not strong symbol
is found at link time.

Differential Revision: https://reviews.llvm.org/D35029

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307348 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed