Track the typical #ifndef/#define/#endif usage in header files to see if
the macro is an include guard. If so, store it in the Document. No
behavioural change, just recording the name.

This can be used in the future to track if a file needs to be re-parsed
when a macro changes: if it was used in the file, and not defined in it
nor being the include-guard, a file should be re-preprocessed and
re-parsed.

It can also be used to check if two files have the same include guard.

Change-Id: I2715f529997a7b24a11bdbc6150652e2669f1a46
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>