Add cxx119to_string to ignored symbol list

Looks like more fallout from the move to C++17 on Linux.
Adding the symbol from the C++ stdlib to the ignore list.

Author: etcwilde

test/stdlib/symbol-visibility-linux.test-sh

@@ -243,6 +243,7 @@
 // RUN:             -e _ZStplIcSt11char_traitsIcESaIcEENSt7__cxx1112basic_stringIT_T0_T1_EES5_RKS8_ \
 // RUN:             -e _ZStplIcSt11char_traitsIcESaIcEESbIT_T0_T1_ERKS6_S8_ \
 // RUN:             -e _ZNSt6vectorIjSaIjEE12emplace_backIJjEEERjDpOT_ \
+// RUN:             -e _ZNSt7__cxx119to_stringEi \
 // RUN:   > %t/swiftCore-all.txt
 // RUN: %llvm-nm --defined-only --extern-only --no-weak %platform-dylib-dir/%target-library-name(swiftCore) > %t/swiftCore-no-weak.txt
 // RUN: diff -u %t/swiftCore-all.txt %t/swiftCore-no-weak.txt
@@ -484,6 +485,7 @@
 // RUN:             -e _ZNSt6vectorIjSaIjEE12emplace_backIJjEEERjDpOT_ \
 // RUN:             -e _ZNSt6vectorImSaImEE12emplace_backIJiEEERmDpOT_ \
 // RUN:             -e _ZNSt6vectorImSaImEE12emplace_backIJmEEERmDpOT_ \
+// RUN:             -e _ZNSt7__cxx119to_stringEi \
 // RUN:   > %t/swiftRemoteMirror-all.txt
 // RUN: %llvm-nm --defined-only --extern-only --no-weak %platform-dylib-dir/%target-library-name(swiftRemoteMirror) > %t/swiftRemoteMirror-no-weak.txt
 // RUN: diff -u %t/swiftRemoteMirror-all.txt %t/swiftRemoteMirror-no-weak.txt