lld: unquote possibly quoted EXTERN("symbol") entry in linker script.

gold accepts quoted strings. binutils requires quoted strings for some
kinds of symbols, e.g.:

  it accepts quoted symbols with @ in name:

  $ echo 'EXTERN("__libc_start_main@@GLIBC_2.2.5")' > a.script
  $ g++ a.script
  /usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o: In function `_start':
  (.text+0x20): undefined reference to `main'
  collect2: error: ld returned 1 exit status

  but rejects them if unquoted:

  $ echo 'EXTERN(__libc_start_main@@GLIBC_2.2.5)' > a.script
  $ g++ a.script
  a.script: file not recognized: File format not recognized
  collect2: error: ld returned 1 exit status

To maintain compatibility with existing linker scripts support quoted
strings in lld as well.

Patch by Lucian Adrian Grijincu.

Differential Revision: https://reviews.llvm.org/D57987

llvm-svn: 353756

lld/ELF/ScriptParser.cpp

@@ -329,7 +329,7 @@ void ScriptParser::readEntry() {
 void ScriptParser::readExtern() {
   expect("(");
   while (!errorCount() && !consume(")"))
-    Config->Undefined.push_back(next());
+    Config->Undefined.push_back(unquote(next()));
 }
 
 void ScriptParser::readGroup() {