[dsymutil] Fix handling of aliases to private external symbols

dsymutil was incorrectly ignoring aliases to private extern symbols in
the MachODebugMapParser. This resulted in spurious warnings about not
being able to find symbols.

rdar://49652389

Differential revision: https://reviews.llvm.org/D89444

(cherry picked from commit f9fb9da)

Author: JDevlieghere

llvm/test/tools/dsymutil/ARM/private-extern-alias.test

@@ -0,0 +1,29 @@
+$ cat private_extern.c
+__attribute__((visibility("hidden")))
+int* foo() {
+  int i = 10;
+  volatile int* j = &i;
+  return j;
+}
+
+int* bar() {
+  return foo();
+}
+
+$ cat main.c
+int* bar();
+int main() {
+  return *bar();
+}
+
+$ cat alias_list
+_foo _baz
+
+$ xcrun --sdk iphoneos clang -g private_extern.c -c -o private_extern.o -target arm64-apple-ios14.0
+$ xcrun --sdk iphoneos clang -g main.c -c -o main.o -target arm64-apple-ios14.0
+$ xcrun --sdk iphoneos clang private_extern.o main.o -target arm64-apple-ios14.0 -o private_extern.out -Xlinker -alias_list -Xlinker alias_list
+
+RUN: dsymutil -oso-prepend-path %p/../Inputs %p/../Inputs/private/tmp/private_extern/private_extern.out -o %t.dSYM --verbose 2>&1 | FileCheck %s
+CHECK-NOT: could not find object file symbol for symbol _baz
+CHECK: { sym: _foo, objAddr: 0x0000000000000000, binAddr: 0x0000000100007F58, size: 0x00000020 }
+CHECK: { sym: _baz, objAddr: 0x0000000000000000, binAddr: 0x0000000100007F58, size: 0x00000000 }

llvm/test/tools/dsymutil/Inputs/private/tmp/private_extern/main.o

@@ -562,7 +562,9 @@ void MachODebugMapParser::loadMainBinarySymbols(
       continue;
     }
     Section = *SectionOrErr;
-    if (Section == MainBinary.section_end() || Section->isText())
+    if ((Section == MainBinary.section_end() || Section->isText()) &&
+        !(SymType &
+          MachO::N_PEXT)) // Alias to non-external (was a private external)
       continue;
     uint64_t Addr = cantFail(Sym.getValue());
     Expected<StringRef> NameOrErr = Sym.getName();