[WebAssembly] Re-enable disabled debug value test

Summary:
This test was resulting in asan/valgrind failures due to undefined
DWARF register mappings for WebAssembly, and was disabled in r278495.
These have been resolved.

Reviewers: sunfish, dschuff

Subscribers: bkramer, llvm-commits, jfb

Differential Revision: https://reviews.llvm.org/D23459

llvm-svn: 278576