[llvm][dsymutil] Add DW_TAG_imported_declaration to accelerator table

**Summary**

After this patch, `dsymutil` will preserve `DW_TAG_imported_declarations`
entries in accelerator tables.

This allows consumers to resolve imported declarations even on
executables processsed through dsymutil.

This helps consumers, particularly LLDB's expression evaluator,
to resolve imported declarations (i.e., useful for namespace aliases
in C++) more efficiently.

**Testing**

* Added unit-test

Differential Revision: https://reviews.llvm.org/D143458

Author: Michael137

llvm/lib/DWARFLinker/DWARFLinker.cpp

@@ -1589,6 +1589,8 @@ DIE *DWARFLinker::DIECloner::cloneDIE(const DWARFDie &InputDIE,
     if (!AttrInfo.Name)
       AttrInfo.Name = StringPool.getEntry("(anonymous namespace)");
     Unit.addNamespaceAccelerator(Die, AttrInfo.Name);
+  } else if (Tag == dwarf::DW_TAG_imported_declaration && AttrInfo.Name) {
+    Unit.addNamespaceAccelerator(Die, AttrInfo.Name);
   } else if (isTypeTag(Tag) && !AttrInfo.IsDeclaration &&
              getDIENames(InputDIE, AttrInfo, StringPool) && AttrInfo.Name &&
              AttrInfo.Name.getString()[0]) {

llvm/test/tools/dsymutil/ARM/accel-imported-declarations.test

@@ -0,0 +1,47 @@
+RUN: dsymutil -accelerator=Dwarf %p/../Inputs/accel-imported-declaration.macho-arm64 -o %t.dwarf.dSYM
+RUN: dsymutil -accelerator=Apple %p/../Inputs/accel-imported-declaration.macho-arm64 -o %t.apple.dSYM
+
+RUN: llvm-dwarfdump -v %t.dwarf.dSYM | FileCheck %s -check-prefixes=DWARF,COMMON
+RUN: llvm-dwarfdump -v %t.apple.dSYM | FileCheck %s -check-prefixes=APPLE,COMMON
+
+COMMON: .debug_info contents
+COMMON: {{.*}}DW_TAG_namespace
+COMMON:   DW_AT_name{{.*}}"A"
+COMMON: {{.*}}DW_TAG_namespace
+COMMON:   DW_AT_name{{.*}}"B"
+COMMON: [[NAMESPACE:0x[0-9a-f]*]]:{{.*}}DW_TAG_namespace
+COMMON:   DW_AT_name{{.*}}"C"
+COMMON: [[IMPORTED:0x[0-9a-f]*]]:{{.*}}DW_TAG_imported_declaration
+COMMON:   DW_AT_name{{.*}}"C"
+
+DWARF:      .debug_names contents:
+DWARF:      Bucket 0 [
+DWARF-NEXT:   Name {{.*}} {
+DWARF-NEXT:     Hash: {{.*}}
+DWARF-NEXT:     String: {{.*}} "C"
+DWARF-NEXT:     Entry {{.*}} {
+DWARF-NEXT:       Abbrev: {{.*}}
+DWARF-NEXT:       Tag: DW_TAG_namespace
+DWARF-NEXT:       DW_IDX_die_offset: [[NAMESPACE]]
+DWARF-NEXT:     }
+DWARF-NEXT:     Entry {{.*}} {
+DWARF-NEXT:       Abbrev: {{.*}}
+DWARF-NEXT:       Tag: DW_TAG_imported_declaration
+DWARF-NEXT:       DW_IDX_die_offset: [[IMPORTED]]
+DWARF-NEXT:     }
+DWARF-NEXT:   }
+
+APPLE:      .apple_namespaces contents:
+APPLE:      Bucket 1 [
+APPLE-NEXT:   Hash {{.*}} [
+APPLE-NEXT:     Name@{{.*}} {
+APPLE-NEXT:       String: {{.*}} "C"
+APPLE-NEXT:       Data 0 [
+APPLE-NEXT:         Atom[0]: [[NAMESPACE]]
+APPLE-NEXT:       ]
+APPLE-NEXT:       Data 1 [
+APPLE-NEXT:         Atom[0]: [[IMPORTED]]
+APPLE-NEXT:       ]
+APPLE-NEXT:     }
+APPLE-NEXT:   ]
+APPLE-NEXT: ]

llvm/test/tools/dsymutil/Inputs/accel-imported-declaration.cpp

@@ -0,0 +1,17 @@
+// Compiled on macOS using:
+// clang++ -c -std=c++2a -gdwarf-4 -O0 -o accel-imported-declaration.macho-arm64.o
+
+namespace A {
+namespace B {
+namespace C {
+int a = -1;
+} // namespace C
+} // namespace B
+
+namespace C = B::C;
+
+using namespace B::C;
+using B::C::a;
+} // namespace A
+
+int main() { return A::a; }