Fix a dynamic symbol lookup issue in interpreter mode.

This was a very subtle bug, which occurred only in interpreter mode under Linux.
The actual name of the symbol should be artificially prefixed by an underscore,
because this underscore is stripped during a symbol lookup in interpreter mode.
If this is not done, then a reference to variable "_x" is being resolved as a reference
to the function "x"!

lib/IRGen/IRGenModule.cpp

@@ -595,8 +595,14 @@ llvm::Constant *swift::getWrapperFn(llvm::Module &Module,
                                          ARGS, ATTRS)                          \
   llvm::Constant *IRGenModule::get##ID##Fn() {                                 \
     using namespace RuntimeConstants;                                          \
-    return getWrapperFn(Module, ID##Fn, #NAME, STR(SYMBOL_NAME), CC, RETURNS,  \
+    /* Add an underscore in JIT mode under Linux to enable proper symbol       \
-                        ARGS, ATTRS);                                          \
+     * lookup */                                                               \
+    const char *Symbol =                                                       \
+        (Opts.UseJIT && TargetInfo.OutputObjectFormat == llvm::Triple::ELF)    \
+            ? "_" STR(SYMBOL_NAME)                                             \
+            : STR(SYMBOL_NAME);                                                \
+    return getWrapperFn(Module, ID##Fn, #NAME, Symbol, CC, RETURNS, ARGS,      \
+                        ATTRS);                                                \
   }
 
 #include "swift/Runtime/RuntimeFunctions.def"