[ELF] - Add test to check we don't crash when tracing reserved symbol.

This is to test the following line of code:
https://github.com/llvm-mirror/lld/blob/master/ELF/SymbolTable.cpp#L601

Without that line linker would crash.

llvm-svn: 336345

lld/test/ELF/trace-symbols.s

@@ -76,6 +76,9 @@
 # RUN:   FileCheck -check-prefix=STARTLIB %s
 # STARTLIB: trace-symbols.s.tmp1: reference to bar
 
+## Check we do not crash when trying to trace special symbol.
+# RUN: not ld.lld -trace-symbol=_end %t -o /dev/null
+
 .hidden hsymbol
 .globl	_start
 .type	_start, @function