Add offsetof support to expression evaluator.

[teemperor]

Summary

We currently don't support offsetof in the expression evaluator as it is implemented as a macro
(which then calls __builtin_offsetof) in stddef.h. The best solution would be to include that
header (or even better, import Clang's builtin module), but header-parsing and
(cross-platform) importing modules is not ready yet.

Until we get this working with modules I would say we add the macro to our existing macro list
as we already do with other macros from stddef.h/stdint.h. We should be able to drop all of them
once we can import the relevant modules by default.

rdar://26040641

[davide]

This is good for now. I would add a comment explaining what you explained in the differential at the beginning of the macro section.
Great work!

[clayborg]

nice!

[shafik]

Thanks for fixing this one!

lldb/packages/Python/lldbsuite/test/lang/c/offsetof/Makefile

+LEVEL = ../../../make
+C_SOURCES := main.c
+include $(LEVEL)/Makefile.rules

lldb/packages/Python/lldbsuite/test/lang/c/offsetof/TestOffsetof.py

+from lldbsuite.test import lldbinline
+
+lldbinline.MakeInlineTest(__file__, globals())

lldb/packages/Python/lldbsuite/test/lang/c/offsetof/main.c

+#include <stdint.h>
+
+struct Foo {
+  int8_t a;
+  int16_t b;
+};
+
+int main (int argc, char const *argv[]) {
+    struct Foo f;
+    return f.a; //% self.expect("expr offsetof(Foo, a)", substrs = ['= 0'])
+                //% self.expect("expr offsetof(Foo, b)", substrs = ['= 2'])
+}

lldb/packages/Python/lldbsuite/test/lang/cpp/offsetof/Makefile

+LEVEL = ../../../make
+CXX_SOURCES := main.cpp
+include $(LEVEL)/Makefile.rules

lldb/packages/Python/lldbsuite/test/lang/cpp/offsetof/TestOffsetofCpp.py

+from lldbsuite.test import lldbinline
+
+lldbinline.MakeInlineTest(__file__, globals())

lldb/packages/Python/lldbsuite/test/lang/cpp/offsetof/main.cpp

+#include <cstdint>
+
+class Base {
+  int32_t a;
+};
+class Class1 : Base {
+public:
+  int32_t b;
+};
+
+class EmptyBase {
+};
+class Class2 : EmptyBase {
+public:
+  int32_t b;
+};
+
+int main(int argc, char **argv) {
+  Class1 c1;
+  Class2 c2;
+  //% self.expect("expr offsetof(Base, a)", substrs=["= 0"])
+  //% self.expect("expr offsetof(Class1, b)", substrs=["= 4"])
+  //% self.expect("expr offsetof(Class2, b)", substrs=["= 0"])
+  return c1.b + c2.b;
+}

lldb/source/Plugins/ExpressionParser/Clang/ClangExpressionSourceCode.cpp

 #include "lldb/Target/Platform.h"
 #include "lldb/Target/StackFrame.h"
 #include "lldb/Target/Target.h"
 #include "lldb/Utility/StreamString.h"
 
 using namespace lldb_private;
 
 const char *ClangExpressionSourceCode::g_expression_prefix = R"(
+#ifndef offsetof
+#define offsetof(t, d) __builtin_offsetof(t, d)
+#endif
 #ifndef NULL
 #define NULL (__null)
 #endif
 #ifndef Nil
 #define Nil (__null)
 #endif
 #ifndef nil
 #define nil (__null)