[WebAssembly] Section symbols must have local binding

Summary: Also test for symbols information in test/MC/WebAssembly/debug-info.ll.

Subscribers: jfb, dschuff, jgravelle-google, aheejin, sunfish, JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D46160

llvm-svn: 331005