Handle demangling prefix @__swiftmacro_ used for filenames.

Author: DougGregor

docs/ABI/Mangling.rst

@@ -9,6 +9,7 @@ Mangling
 ::
 
   mangled-name ::= '$s' global  // Swift stable mangling
+  mangled-name ::= '@__swiftmacro_' global // Swift mangling for filenames
   mangled-name ::= '_T0' global // Swift 4.0
   mangled-name ::= '$S' global  // Swift 4.2
 

lib/Demangling/Demangler.cpp

@@ -177,7 +177,9 @@ int swift::Demangle::getManglingPrefixLength(llvm::StringRef mangledName) {
   llvm::StringRef prefixes[] = {
     /*Swift 4*/   "_T0",
     /*Swift 4.x*/ "$S", "_$S",
-    /*Swift 5+*/  "$s", "_$s"};
+    /*Swift 5+*/  "$s", "_$s",
+    /*Swift 5+ for filenames*/ "@__swiftmacro_",
+  };
 
   // Look for any of the known prefixes
   for (auto prefix : prefixes) {

test/Demangle/Inputs/manglings.txt

@@ -452,3 +452,4 @@ $s4main4TestV1xSivpfaAA4Flag ---> runtime attribute generator of main.Test.x : S
 $s4main4TestAaBVmvpfaAA9OtherFlag ---> runtime attribute generator of main.Test : main.Test.Type for attribute = main.OtherFlag
 $s9MacroUser13testStringify1a1bySi_SitF9stringifyfMf1_ ---> freestanding macro expansion #3 of stringify in MacroUser.testStringify(a: Swift.Int, b: Swift.Int) -> ()
 $s9MacroUser016testFreestandingA9ExpansionyyF4Foo3L_V23bitwidthNumberedStructsfMf_6methodfMu0_ ---> unique name #2 of method in freestanding macro expansion #1 of bitwidthNumberedStructs in Foo3 #1 in MacroUser.testFreestandingMacroExpansion() -> ()
+@__swiftmacro_1a13testStringifyAA1bySi_SitF9stringifyfMf_  ---> freestanding macro expansion #1 of stringify in a.testStringify(a: Swift.Int, b: Swift.Int) -> ()

tools/swift-demangle/swift-demangle.cpp

@@ -271,7 +271,7 @@ static bool findMaybeMangled(llvm::StringRef input, llvm::StringRef &match) {
   } state = Start;
   const char *matchStart = nullptr;
 
-  // Find _T, $S, $s, _$S, _$s followed by a valid mangled string
+  // Find _T, $S, $s, _$S, _$s, @__swift_ followed by a valid mangled string
   while (ptr < end) {
     switch (state) {
     case Start:
@@ -286,6 +286,12 @@ static bool findMaybeMangled(llvm::StringRef input, llvm::StringRef &match) {
           state = SeenDollar;
           matchStart = ptr - 1;
           break;
+        } else if (ch == '@' &&
+                   llvm::StringRef(ptr, end - ptr).startswith("__swiftmacro_")){
+          matchStart = ptr - 1;
+          ptr = ptr + strlen("__swift_");
+          state = FoundPrefix;
+          break;
         }
       }
       break;