[Support] Add a C-API function to create a StringError instance.

This will allow C API clients to return errors from callbacks. This
functionality will be used in upcoming Orc C-bindings functions.
diff --git a/llvm/include/llvm-c/Error.h b/llvm/include/llvm-c/Error.h
index 92f81bf..bc702ac 100644
--- a/llvm/include/llvm-c/Error.h
+++ b/llvm/include/llvm-c/Error.h
@@ -62,6 +62,11 @@
  */
 LLVMErrorTypeId LLVMGetStringErrorTypeId(void);
 
+/**
+ * Create a StringError.
+ */
+LLVMErrorRef LLVMCreateStringError(const char *ErrMsg);
+
 LLVM_C_EXTERN_C_END
 
 #endif