[libc++abi] Fix issue when building the demangler in C++11

Captures with an initializer only work in C++14. This broke the C++11
CI but wasn't noticed because our CI was down.

Author: ldionne

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -2632,7 +2632,8 @@ template<typename NodeT> struct NodeKind;
 #include "ItaniumNodes.def"
 
 inline bool NodeArray::printAsString(OutputBuffer &OB) const {
-  auto Fail = [&OB, StartPos = OB.getCurrentPosition()] {
+  auto StartPos = OB.getCurrentPosition();
+  auto Fail = [&OB, StartPos] {
     OB.setCurrentPosition(StartPos);
     return false;
   };

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -2632,7 +2632,8 @@ template<typename NodeT> struct NodeKind;
 #include "ItaniumNodes.def"
 
 inline bool NodeArray::printAsString(OutputBuffer &OB) const {
-  auto Fail = [&OB, StartPos = OB.getCurrentPosition()] {
+  auto StartPos = OB.getCurrentPosition();
+  auto Fail = [&OB, StartPos] {
     OB.setCurrentPosition(StartPos);
     return false;
   };