[WebAssembly] Fix __builtin_wasm_tls_base intrinsic

Summary:
Properly generate the outchain for the `__builtin_wasm_tls_base` intrinsic.

Also marked the intrinsic pure, per @sunfish's suggestion.

Reviewers: tlively, aheejin, sbc100, sunfish

Reviewed By: tlively

Subscribers: dschuff, jgravelle-google, hiraditya, cfe-commits, llvm-commits, sunfish

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D64949

llvm-svn: 366499