Fix C++ demangling for _BitInt type

    * FE expects _BitInt to be available for substitution;
      ensure DB<n> is added to Subs in ItaniumDemangle.h
    * Add a test case to libcxxabi
    * Copy changed files to llvm/include/llvm/

Author: lfmeadow

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -4468,7 +4468,9 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
         return nullptr;
       if (!consumeIf('_'))
         return nullptr;
-      return make<BitIntType>(Size, Signed);
+      // The FE expects this to be available for Substitution
+      Result = make<BitIntType>(Size, Signed);
+      break;
     }
     //                ::= Di   # char32_t
     case 'i':

libcxxabi/test/DemangleTestCases.inc

@@ -6,6 +6,7 @@
 {"_Z1fDU10_", "f(unsigned _BitInt(10))"},
 {"_Z1fIfEvDUstPT__", "void f<float>(unsigned _BitInt(sizeof (float*)))"},
 {"_Z1fIiEvDBstPT__", "void f<int>(_BitInt(sizeof (int*)))"},
+{"_Z6myfuncRDB8_S0_", "myfunc(_BitInt(8)&, _BitInt(8)&)"},
 {"_Z4testI1A1BE1Cv", "C test<A, B>()"},
 {"_Z4testI1A1BET0_T_S3_", "B test<A, B>(A, A)"},
 {"_ZN1SgtEi", "S::operator>(int)"},

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -4468,7 +4468,9 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
         return nullptr;
       if (!consumeIf('_'))
         return nullptr;
-      return make<BitIntType>(Size, Signed);
+      // The FE expects this to be available for Substitution
+      Result = make<BitIntType>(Size, Signed);
+      break;
     }
     //                ::= Di   # char32_t
     case 'i':

llvm/include/llvm/Testing/Demangle/DemangleTestCases.inc

@@ -6,6 +6,7 @@
 {"_Z1fDU10_", "f(unsigned _BitInt(10))"},
 {"_Z1fIfEvDUstPT__", "void f<float>(unsigned _BitInt(sizeof (float*)))"},
 {"_Z1fIiEvDBstPT__", "void f<int>(_BitInt(sizeof (int*)))"},
+{"_Z6myfuncRDB8_S0_", "myfunc(_BitInt(8)&, _BitInt(8)&)"},
 {"_Z4testI1A1BE1Cv", "C test<A, B>()"},
 {"_Z4testI1A1BET0_T_S3_", "B test<A, B>(A, A)"},
 {"_ZN1SgtEi", "S::operator>(int)"},