[WebAssembly] Fix wasm backend compilation on gcc 5.4: variable name cannot match class

Summary:
gcc does not like

const Region *Region;

It wants a different name for the variable.

Is there a better convention for what name to use in such a case?

Reviewers: sbc100, aheejin

Subscribers: aheejin, jgravelle-google, dschuff, llvm-commits

Differential Revision: https://reviews.llvm.org/D50472

Patch by Alon Zakai (kripken)

llvm-svn: 339398