DebugInfo: Don't simplify template names using _BitInt(N)

_BitInt(N) only encodes the byte size in DWARF, not the bit size, so
can't be reconstituted.

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -5032,6 +5032,15 @@ struct ReconstitutableType : public RecursiveASTVisitor<ReconstitutableType> {
     Reconstitutable = false;
     return false;
   }
+  bool VisitType(Type *T) {
+    // _BitInt(N) isn't reconstitutable because the bit width isn't encoded in
+    // the DWARF, only the byte width.
+    if (T->isBitIntType()) {
+      Reconstitutable = false;
+      return false;
+    }
+    return true;
+  }
   bool TraverseEnumType(EnumType *ET) {
     // Unnamed enums can't be reconstituted due to a lack of column info we
     // produce in the DWARF, so we can't get Clang's full name back.

clang/test/CodeGenCXX/debug-info-simple-template-names.cpp

@@ -105,4 +105,10 @@ void f() {
 
   f3<t1>();
   // CHECK: !DISubprogram(name: "_STNf3|<t1>",
+  
+  f1<_BitInt(3)>();
+  // CHECK: !DISubprogram(name: "f1<_BitInt(3)>",
+
+  f1<const unsigned _BitInt(5)>();
+  // CHECK: !DISubprogram(name: "f1<const unsigned _BitInt(5)>",
 }

cross-project-tests/debuginfo-tests/clang_llvm_roundtrip/simplified_template_names.cpp

@@ -316,6 +316,8 @@ int main() {
   f1<void(t8)>();
   operator_not_really<int>();
   t12 v4;
+  f1<_BitInt(3)>();
+  f1<const unsigned _BitInt(5)>();
 }
 void t8::mem() {
   struct t7 { };