[clang][DebugInfo] Mark _BitInt's as reconstitutable when emitting -gsimple-template-names

Depends on:
* #168382

As of recent, LLVM includes the bit-size as a `DW_AT_bit_size` (and as part of `DW_AT_name`) of `_BitInt`s in DWARF. This allows us to mark `_BitInt`s as "reconstitutable" when compiling with `-gsimple-template-names`. We still only omits template parameters that are `<= 64` bit wide. So support `_BitInt`s larger than 64 bits is not part of this patch.

Author: Michael137

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -5863,15 +5863,6 @@ 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, bool = false) {
     // 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/DebugInfo/CXX/simple-template-names.cpp

@@ -114,12 +114,28 @@ void f() {
 
   f3<t1>();
   // CHECK: !DISubprogram(name: "_STN|f3|<t1>",
-  
+
   f1<_BitInt(3)>();
-  // CHECK: !DISubprogram(name: "f1<_BitInt(3)>",
+  // CHECK: !DISubprogram(name: "_STN|f1|<_BitInt(3)>",
 
   f1<const unsigned _BitInt(5)>();
-  // CHECK: !DISubprogram(name: "f1<const unsigned _BitInt(5)>",
+  // CHECK: !DISubprogram(name: "_STN|f1|<const unsigned _BitInt(5)>",
+
+  f1<_BitInt(120)>();
+  // CHECK: !DISubprogram(name: "_STN|f1|<_BitInt(120)>",
+
+  f1<const unsigned _BitInt(120)>();
+  // CHECK: !DISubprogram(name: "_STN|f1|<const unsigned _BitInt(120)>",
+
+  f2<_BitInt(2), 1>();
+  // CHECK: !DISubprogram(name: "_STN|f2|<_BitInt(2), (_BitInt(2))1>",
+
+  f2<_BitInt(64), 12>();
+  // CHECK: !DISubprogram(name: "_STN|f2|<_BitInt(64), (_BitInt(64))12>",
+
+  // FIXME: make block forms reconstitutable
+  f2<_BitInt(65), 1>();
+// CHECK: !DISubprogram(name: "f2<_BitInt(65), (_BitInt(65))1>",
 
   // Add a parameter just so this differs from other attributed function types
   // that don't mangle differently.