[ELF,test] Add static-pie test related to demoted lazy symbol

The reverted
1a4d6de
("[ELF] Remove redundant isExported computation")
had incorrect
```
+        if (sym->includeInDynsym(ctx))
+          sym->isExported = true;
```

causing undefined weak symbols (defined in archives, demoted; e.g.
__cxa_finalize) to be exported for static-pie.

Add a regression test for this corner case. The issue actually exposed
another issue related to includeInDynsym, which has been fixed by
f10441a.

Author: MaskRay

lld/test/ELF/weak-undef-lib.s

@@ -17,6 +17,11 @@
 # CHECK-NEXT: Other: 0
 # CHECK-NEXT: Section: Undefined
 
+# RUN: ld.lld -pie -o %t %t1.o --start-lib %t2.o --no-dynamic-linker
+# RUN: llvm-readelf --dyn-syms %t | FileCheck %s --check-prefix=STATICPIE
+
+# STATICPIE: Symbol table '.dynsym' contains 1
+
 ## -u specifies a STB_DEFAULT undefined symbol, so the definition from %t2.o is
 ## fetched.
 # RUN: ld.lld -u foo %t1.o --start-lib %t2.o -o %t1