[WebAssembly] Use the "wasm32-wasi" triple in tests

Similar to https://reviews.llvm.org/D61334, update clang tests to use the
"wasm32-wasi" triple, removing the "-musl" environment and omitting the
"-unknown" vendor.

Differential Revision: https://reviews.llvm.org/D61338

Reviewer: sbc100
llvm-svn: 359630