Fix issues in TCling::IsLoaded()

Use HeaderSearch::LookupFile() instead of Preprocessor::LookupFile(), which requires a proper lexer to be set, and crashes when it is not the case
ignore generated file: *_rdict.pcm, *_dictContent.h and *_dictUmbrella.h.
Finally remove TFile::UseCache() which was marked for removal by v5-32-00.
mark ReAlloc(void*,size_t) as obsolete for v6-02-00 as it is non-safe.
Forward to the const getter.
Use the ASTContext stored in the Transaction.
The ASTContext is a ref.
Use the non-asserting getDeclName as it is used all over the DeclPrinter.cpp
Constify.
Iterating over the AST might trigger deserialization.
Add another sanity check. It will cry whenever we extend the enum.
Order.
Implement pretty print of DelayCallInfo. Constify.
Fix getters, ending up in infinite recursion...
There is no need of checking the size of the transaction.
Print the decls coming from deserialization, too.
Move codegenning of deserialized declarations in the IncrementalParser.

We had two different codepath for codegeneration. The first one was for
declarations that came from text and the second one for declarations that came
from 'ExternalSource' such as PCH. In the latter case we had to do some
extra equilibristics to emulate a PCM and ignore generating code for decls with
static storage, because it will clash with the loading of the library that already
contains them.

The advantage of the new implementation is that instead of communicating the
deserialized decls to ROOT via some fishy deserialization listeners we can use
the cling::Transaction as we use it for decls that came from code.
Remove now redundant code
Get rid of the unused callbacks.
Finally fix that infinite const recursion.