[WebAssembly] Use "long long" for int_fast64_t and int_least64_t on wasm64

This makes int_fast64_t and int_least64_t the same type as int64_t, and
eliminates a difference between wasm32 and wasm64.

Differential Revision: http://reviews.llvm.org/D12861


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247622 91177308-0d34-0410-b5e6-96231b3b80d8