Track undefined sym in the symtab. Remove hack for missing relocs against undef.

tests_lit/llvm2ice_tests/elf_container.ll

 ; Tests that we generate an ELF container with fields that make sense,
 ; cross-validating against llvm-mc.
 
 ; For the integrated ELF writer, we can't pipe the output because we need
 ; to seek backward and patch up the file headers. So, use a temporary file.
 ; RUN: %p2i -i %s --args -O2 --verbose none -elf-writer -o %t \
 ; RUN:   && llvm-readobj -file-headers -sections -section-data \
 ; RUN:       -relocations -symbols %t | FileCheck %s
 
 ; RUN: %p2i -i %s --args -O2 --verbose none \
@@ skipping 367 matching lines @@
 ; CHECK:   }
 
 
 ; CHECK: Relocations [
 ; CHECK:   Section ({{[0-9]+}}) .rel.text {
 ; CHECK:     0x4 R_386_32 .L$float$0 0x0
 ; CHECK:     0xC R_386_32 .L$float$1 0x0
 ; CHECK:     0x24 R_386_32 .L$double$0 0x0
 ; CHECK:     0x2C R_386_32 .L$double$1 0x0
 ; CHECK:     0x34 R_386_32 .L$double$2 0x0
-; The set of relocations between llvm-mc and integrated elf-writer
-; are different. The integrated elf-writer does not yet handle
-; external/undef functions like memcpy.  Also, it does not resolve internal
-; function calls and instead writes out the relocation. However, there's
-; probably some function call so check for a PC32 relocation at least.
-; CHECK:     0x{{.*}} R_386_PC32
+; CHECK:     0x{{.*}} R_386_PC32 memcpy
+; CHECK:     0x{{.*}} R_386_PC32 memset
 ; CHECK:   }
 ; CHECK:   Section ({{[0-9]+}}) .rel.data {
 ; The set of relocations between llvm-mc and the integrated elf-writer
 ; are different. For local symbols, llvm-mc uses the section + offset within
 ; the section, while the integrated elf-writer refers the symbol itself.
 ; CHECK:     0x10 R_386_32 {{.*}} 0x0
 ; CHECK:     0x14 R_386_32 {{.*}} 0x0
 ; CHECK:     0x18 R_386_32 {{.*}} 0x0
 ; CHECK:   }
 ; CHECK:   Section ({{[0-9]+}}) .rel.rodata {
@@ skipping 206 matching lines @@
 ; CHECK-NEXT:   }
 ; CHECK:        Symbol {
 ; CHECK:          Name: _start
 ; CHECK-NEXT:     Value: 0x{{[1-9A-F][0-9A-F]*}}
 ; CHECK-NEXT:     Size: 0
 ; CHECK-NEXT:     Binding: Global
 ; CHECK-NEXT:     Type: Function
 ; CHECK-NEXT:     Other: 0
 ; CHECK-NEXT:     Section: .text
 ; CHECK-NEXT:   }
+; CHECK:        Symbol {
+; CHECK:          Name: memcpy
+; CHECK-NEXT:     Value: 0x0
+; CHECK-NEXT:     Size: 0
+; CHECK-NEXT:     Binding: Global
+; CHECK-NEXT:     Type: None
+; CHECK-NEXT:     Other: 0
+; CHECK-NEXT:     Section: Undefined
+; CHECK-NEXT:   }
+; CHECK:        Symbol {
+; CHECK:          Name: memset
+; CHECK-NEXT:     Value: 0x0
+; CHECK-NEXT:     Size: 0
+; CHECK-NEXT:     Binding: Global
+; CHECK-NEXT:     Type: None
+; CHECK-NEXT:     Other: 0
+; CHECK-NEXT:     Section: Undefined
+; CHECK-NEXT:   }
 ; CHECK: ]