[ORC] Add an 'intern' method to ExecutionEngine for interning symbol names.

This cuts down on boilerplate by reducing 'ES.getSymbolStringPool().intern(...)'
to 'ES.intern(...)'.

llvm-svn: 343427
diff --git a/llvm/unittests/ExecutionEngine/Orc/LazyCallThroughAndReexportsTest.cpp b/llvm/unittests/ExecutionEngine/Orc/LazyCallThroughAndReexportsTest.cpp
index c3c5703..ebe99d4 100644
--- a/llvm/unittests/ExecutionEngine/Orc/LazyCallThroughAndReexportsTest.cpp
+++ b/llvm/unittests/ExecutionEngine/Orc/LazyCallThroughAndReexportsTest.cpp
@@ -33,7 +33,7 @@
     return;
   }
 
-  auto DummyTarget = ES.getSymbolStringPool().intern("DummyTarget");
+  auto DummyTarget = ES.intern("DummyTarget");
 
   bool DummyTargetMaterialized = false;