ASTMangler: Fix assertion when mangling invalid identifiers

We sometimes need to mangle invalid identifiers, for example when sorting
declarations in EmittedMembersRequest. Make sure this doesn't assert.

Author: slavapestov

include/swift/Demangling/ManglingUtils.h

@@ -192,9 +192,8 @@ void mangleIdentifier(Mangler &M, StringRef ident) {
       // Mangle the sub-string up to the next word substitution (or to the end
       // of the identifier - that's why we added the dummy-word).
       // The first thing: we add the encoded sub-string length.
+      bool first = true;
       M.Buffer << (Repl.StringPos - Pos);
-      assert(!isDigit(ident[Pos]) &&
-             "first char of sub-string may not be a digit");
       do {
         // Update the start position of new added words, so that they refer to
         // the begin of the whole mangled Buffer.
@@ -203,9 +202,16 @@ void mangleIdentifier(Mangler &M, StringRef ident) {
           M.Words[WordsInBuffer].start = M.getBufferStr().size();
           WordsInBuffer++;
         }
+        // Error recovery. We sometimes need to mangle identifiers coming
+        // from invalid code.
+        if (first && isDigit(ident[Pos]))
+          M.Buffer << 'X';
         // Add a literal character of the sub-string.
-        M.Buffer << ident[Pos];
+        else
+          M.Buffer << ident[Pos];
+
         Pos++;
+        first = false;
       } while (Pos < Repl.StringPos);
     }
     // Is it a "real" word substitution (and not the dummy-word)?

validation-test/compiler_crashers/28809-isdigit-ident-pos-first-char-of-sub-string-may-not-be-a-digit.swift renamed to validation-test/compiler_crashers_fixed/28809-isdigit-ident-pos-first-char-of-sub-string-may-not-be-a-digit.swift

@@ -5,6 +5,6 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// REQUIRES: asserts
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+
+// RUN: not %target-swift-frontend %s -emit-ir
 protocol 0{class TextOutputStream