[ELF] - Do not crash when move location counter backward.

PR31335 shows that we do that in next case:
SECTIONS { .text 0x2000 : {. = 0x100 ; *(.text) } }

though documentations says that "If . is used inside a section 
description however, it refers to the byte offset from the start
of that section, not an absolute address. " looks does not work 
as documented in bfd (as mentioned in comments for PR31335).

Until we find out the expected behavior was suggested at least not
to 'crash', what we do after trying to generate huge file.

Differential revision: https://reviews.llvm.org/D27712

llvm-svn: 289782

lld/ELF/LinkerScript.cpp

@@ -464,7 +464,11 @@ template <class ELFT> void LinkerScript<ELFT>::process(BaseCommand &Base) {
   if (auto *AssignCmd = dyn_cast<SymbolAssignment>(&Base)) {
     if (AssignCmd->Name == ".") {
       // Update to location counter means update to section size.
-      Dot = AssignCmd->Expression(Dot);
+      uintX_t Val = AssignCmd->Expression(Dot);
+      if (Val < Dot)
+        error("unable to move location counter backward for: " +
+              CurOutSec->Name);
+      Dot = Val;
       CurOutSec->Size = Dot - CurOutSec->Addr;
       return;
     }

lld/test/ELF/linkerscript/locationcountererr.s

@@ -0,0 +1,9 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { .text 0x2000 : {. = 0x10 ; *(.text) } }" > %t.script
+# RUN: not ld.lld %t --script %t.script -o %t1 2>&1 | FileCheck %s
+# CHECK: unable to move location counter backward for: .text
+
+.globl _start
+_start:
+nop