Add support for triple wasm32-unknown-emscripten (#4672)

Author: JohanEngelen

driver/main.cpp

@@ -908,6 +908,9 @@ void registerPredefinedTargetVersions() {
     VersionCondition::addPredefinedGlobalIdent("WASI");
     VersionCondition::addPredefinedGlobalIdent("CRuntime_WASI");
     break;
+  case llvm::Triple::Emscripten:
+    VersionCondition::addPredefinedGlobalIdent("Emscripten");
+    break;
   default:
     if (triple.getEnvironment() == llvm::Triple::Android) {
       VersionCondition::addPredefinedGlobalIdent("Android");

tests/codegen/wasm_emscripten.d

@@ -0,0 +1,11 @@
+// REQUIRES: target_WebAssembly
+
+// RUN: %ldc -mtriple=wasm32-unknown-emscripten -w -output-ll -of=%t.ll %s
+// RUN: FileCheck %s < %t.ll
+
+// test predefined versions:
+version (Emscripten) {} else static assert(0);
+
+void foo() {}
+
+// CHECK: target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-f128:64-n32:64-S128-ni:1:10:20"