[ELF] - Add support for -nopie

[grimar]

Summary

This is https://llvm.org/bugs/show_bug.cgi?id=30696,

in compare with custom ld that OpenBSD uses, we do not have -pie by default.
That means -nopie can be implemented as noop in lld in theory.

At the same time looks if OpenBSD uses -pie as default we might want to support easy switch for them.
That what this patch do, it introduces way where change of single constant allows to select the default value
and -pie/-nopie options are also supported.

[grimar]

• Use of existent switch argument api instead of new method.

[• rafael]

LGTM

ELF/Driver.cpp

void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->EnableNewDtags = !Args.hasArg(OPT_disable_new_dtags);
   Config->ExportDynamic = Args.hasArg(OPT_export_dynamic);
   Config->FatalWarnings = Args.hasArg(OPT_fatal_warnings);
   Config->GcSections = getArg(Args, OPT_gc_sections, OPT_no_gc_sections, false);
   Config->ICF = Args.hasArg(OPT_icf);
   Config->NoGnuUnique = Args.hasArg(OPT_no_gnu_unique);
   Config->NoUndefinedVersion = Args.hasArg(OPT_no_undefined_version);
   Config->Nostdlib = Args.hasArg(OPT_nostdlib);
-  Config->Pie = Args.hasArg(OPT_pie);
+  Config->Pie = getArg(Args, OPT_pie, OPT_nopie, false);
   Config->PrintGcSections = Args.hasArg(OPT_print_gc_sections);
   Config->Relocatable = Args.hasArg(OPT_relocatable);
   Config->SaveTemps = Args.hasArg(OPT_save_temps);
   Config->Shared = Args.hasArg(OPT_shared);
   Config->Target1Rel = getArg(Args, OPT_target1_rel, OPT_target1_abs, false);
   Config->Threads = Args.hasArg(OPT_threads);
   Config->Trace = Args.hasArg(OPT_trace);
   Config->Verbose = Args.hasArg(OPT_verbose);

ELF/Options.td

def no_gnu_unique: F<"no-gnu-unique">,
   HelpText<"Disable STB_GNU_UNIQUE symbol binding">;
 
 def no_whole_archive: F<"no-whole-archive">,
   HelpText<"Restores the default behavior of loading archive members">;
 
 def noinhibit_exec: F<"noinhibit-exec">,
   HelpText<"Retain the executable output file whenever it is still usable">;
 
+def nopie: F<"nopie">, HelpText<"Do not create a position independent executable">;
+
 def no_undefined: F<"no-undefined">,
   HelpText<"Report unresolved symbols even if the linker is creating a shared library">;
 
 def no_undefined_version: F<"no-undefined-version">,
   HelpText<"Report version scripts that refer undefined symbols">;
 
 def o: JoinedOrSeparate<["-"], "o">, MetaVarName<"<path>">,
   HelpText<"Path to file to write output">;

test/ELF/pie.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1.o
+
+## Default is no PIE.
+# RUN: ld.lld %t1.o -o %t
+# RUN: llvm-readobj -file-headers -sections -program-headers -symbols -r %t \
+# RUN:   | FileCheck %s --check-prefix=NOPIE
+
+## Check -pie.
 # RUN: ld.lld -pie %t1.o -o %t
 # RUN: llvm-readobj -file-headers -sections -program-headers -symbols -r %t | FileCheck %s
 
 ## Test --pic-executable alias
 # RUN: ld.lld --pic-executable %t1.o -o %t
 # RUN: llvm-readobj -file-headers -sections -program-headers -symbols -r %t | FileCheck %s
 
 # CHECK:      ElfHeader {

 # CHECK-NEXT:    MemSize: 112
 # CHECK-NEXT:    Flags [
 # CHECK-NEXT:      PF_R
 # CHECK-NEXT:      PF_W
 # CHECK-NEXT:    ]
 # CHECK-NEXT:    Alignment: 8
 # CHECK-NEXT:  }
 
+## Check -nopie
+# RUN: ld.lld -nopie %t1.o -o %t2
+# RUN: llvm-readobj -file-headers -r %t2 | FileCheck %s --check-prefix=NOPIE
+# NOPIE-NOT: Type: SharedObject
+
 .globl _start
 _start: