[embedded] Teach swift-demangle about prefix, add test

Author: kubamracek

test/Demangle/demangle-embedded.swift

@@ -0,0 +1,4 @@
+; This is not really a Swift source file: -*- Text -*-
+
+; RUN: echo '$e4main8MyStructV3fooyyFAA1XV_Tg5' | swift-demangle | %FileCheck %s
+; CHECK: generic specialization <main.X> of main.MyStruct.foo() -> ()

tools/swift-demangle/swift-demangle.cpp

@@ -346,7 +346,7 @@ static bool findMaybeMangled(llvm::StringRef input, llvm::StringRef &match) {
       while (ptr < end) {
         char ch = *ptr++;
 
-        if (ch == 'S' || ch == 's') {
+        if (ch == 'S' || ch == 's' || ch == 'e') {
           state = FoundPrefix;
           break;
         } else if (ch == '_') {