[clang][Emscripten] Define __unix family of macros

This will allow us to remove these from the downstream
driver:
https://github.com/emscripten-core/emscripten/blob/57270ce8150a5107e591b4e9ec7cbeff0ba7c905/emcc.py#L860-L863

Differential Revision: https://reviews.llvm.org/D108735

Author: sbc100

clang/lib/Basic/Targets/OSTargets.h

@@ -944,6 +944,7 @@ class LLVM_LIBRARY_VISIBILITY EmscriptenTargetInfo
   void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
                     MacroBuilder &Builder) const final {
     WebAssemblyOSTargetInfo<Target>::getOSDefines(Opts, Triple, Builder);
+    DefineStd(Builder, "unix", Opts);
     Builder.defineMacro("__EMSCRIPTEN__");
     if (Opts.POSIXThreads)
       Builder.defineMacro("__EMSCRIPTEN_PTHREADS__");

clang/test/Preprocessor/init.c

@@ -1847,8 +1847,10 @@
 // WEBASSEMBLY-NEXT:#define __clang_version__ "{{.*}}"
 // WEBASSEMBLY-NEXT:#define __clang_wide_literal_encoding__ {{.*}}
 // WEBASSEMBLY-NEXT:#define __llvm__ 1
-// WEBASSEMBLY-NOT:#define __unix
-// WEBASSEMBLY-NOT:#define __unix__
+// WEBASSEMBLY-WASI-NOT:#define __unix
+// WEBASSEMBLY-WASI-NOT:#define __unix__
+// EMSCRIPTEN-NEXT:#define __unix 1
+// EMSCRIPTEN-NEXT:#define __unix__ 1
 // WEBASSEMBLY-WASI-NEXT:#define __wasi__ 1
 // WEBASSEMBLY-NOT:#define __wasm_simd128__
 // WEBASSEMBLY-NOT:#define __wasm_simd256__
@@ -1863,6 +1865,8 @@
 // WEBASSEMBLY64-NEXT:#define __wasm64 1
 // WEBASSEMBLY64-NEXT:#define __wasm64__ 1
 // WEBASSEMBLY-NEXT:#define __wasm__ 1
+// EMSCRIPTEN:#define unix 1
+// WEBASSEMBLY-WASI-NOT:#define unix 1
 // WEBASSEMBLY-CXX-NOT:_REENTRANT
 // WEBASSEMBLY-CXX-NOT:__STDCPP_THREADS__
 // WEBASSEMBLY-CXX-ATOMICS:#define _REENTRANT 1