[lldb] Replace empty ctor en dtor bodies with =default (NFC)

Use = default instead of empty constructor and destructor bodies in the
API layer.
diff --git a/lldb/source/API/SystemInitializerFull.cpp b/lldb/source/API/SystemInitializerFull.cpp
index 894df07..4593bd3 100644
--- a/lldb/source/API/SystemInitializerFull.cpp
+++ b/lldb/source/API/SystemInitializerFull.cpp
@@ -112,9 +112,9 @@
 
 using namespace lldb_private;
 
-SystemInitializerFull::SystemInitializerFull() {}
+SystemInitializerFull::SystemInitializerFull() = default;
 
-SystemInitializerFull::~SystemInitializerFull() {}
+SystemInitializerFull::~SystemInitializerFull() = default;
 
 #define LLDB_PROCESS_AArch64(op) op(ABIAArch64);
 #define LLDB_PROCESS_ARM(op) op(ABIARM);