Fix demangling template parameter packs (#414)

* Fix demangling template parameter packs

Clang 4.0.1-10 and gcc 7.3.0 both mangle the function "void add<int>(int)" as
"_Z3addIJiEEvDpT_".  The template parameter pack is of the form
J <template-arg>* E

The opening character for a param pack could be either I or J, as libiberty
follows [1].  This change simply adds the J case.

[1] https://github.com/gcc-mirror/gcc/blob/fbd263526ad105a953fd51d9f7bca2c3f268cf82/libiberty/cp-demangle.c#L3209

src/demangle.cc

@@ -1097,10 +1097,11 @@ static bool ParseTemplateArgs(State *state) {
 // <template-arg>  ::= <type>
 //                 ::= <expr-primary>
 //                 ::= I <template-arg>* E        # argument pack
+//                 ::= J <template-arg>* E        # argument pack
 //                 ::= X <expression> E
 static bool ParseTemplateArg(State *state) {
   State copy = *state;
-  if (ParseOneCharToken(state, 'I') &&
+  if ((ParseOneCharToken(state, 'I') || ParseOneCharToken(state, 'J')) &&
       ZeroOrMore(ParseTemplateArg, state) &&
       ParseOneCharToken(state, 'E')) {
     return true;

src/demangle_unittest.txt

@@ -135,3 +135,7 @@ _ZlsRSoRKSs	operator<<()
 _ZngILi42EEvN1AIXplT_Li2EEE1TE	operator-<>()
 _ZplR1XS0_	operator+()
 _Zrm1XS_	operator%()
+
+# Template argument packs can start with I or J.
+_Z3addIIiEEvDpT_	add<>()
+_Z3addIJiEEvDpT_	add<>()