[WebAssembly] Support CFI for WebAssembly target

Summary: This patch implements CFI for WebAssembly. It modifies the
LowerTypeTest pass to pre-assign table indexes to functions that are
called indirectly, and lowers type checks to test against the
appropriate table indexes. It also modifies the WebAssembly backend to
support a special ".indidx" assembly directive that propagates the table
index assignments out to the linker.

Patch by Dominic Chen

Differential Revision: https://reviews.llvm.org/D21768

llvm-svn: 277398