WebAssembly: update syntax

Summary:
Follow the same syntax as for the spec repo. Both have evolved slightly
independently and need to converge again.

This, along with wasmate changes, allows me to do the following:

  echo "int add(int a, int b) { return a + b; }" > add.c
  ./out/bin/clang -O2 -S --target=wasm32-unknown-unknown add.c -o add.wack
  ./experimental/prototype-wasmate/wasmate.py add.wack > add.wast
  ./sexpr-wasm-prototype/out/sexpr-wasm add.wast -o add.wasm
  ./sexpr-wasm-prototype/third_party/v8-native-prototype/v8/v8/out/Release/d8 -e "print(WASM.instantiateModule(readbuffer('add.wasm'), {print:print}).add(42, 1337));"

As you'd expect, the d8 shell prints out the right value.

Reviewers: sunfish

Subscribers: jfb, llvm-commits, dschuff

Differential Revision: http://reviews.llvm.org/D13712

llvm-svn: 250480