some more HP-UX cleanup.
Add missing protection in OptimizeBasket.

Some of the branch (because they have been disabled for
example) may not have any entry.  So we now protect
against this case (actually seen in stress).

This fixes ROOT-5410
Avoid spurrious round-trip through string based interface

Add TMethodCall::Init(TFunction*)
to avoid having to go back to the string based interface and
having to do yet another lookup when we already have selected
the function/method we need and thus have already all the
information.
Add missing transaction
Avoid unnecessary lookup and string comparison.

To get the value of TMethodCall::ReturnType rather than doing a fresh
lookup from the string representation of the return type *and* then still
doing string comparison on the partial simplified result,
we know just use the interface of the clang::QualType that is attached
to the underlying function decl.

This also save all the memory cost associated with calling findType.
Avoid unnecessary set up of the parser and memory buffer creation.