Eliminate the branching in QualType::getTypePtr() by providing a

common base for ExtQuals and Type that stores the underlying type
pointer. This results in a 2% performance win for -emit-llvm on a
typical C file, with 1% memory growth in the AST.

Note that there is an API change in this optimization:
QualType::getTypePtr() can no longer be invoked on a NULL
QualType. If the QualType might be NULL, use
QualType::getTypePtrOrNull(). I've audited all uses of getTypePtr() in
the code base and changed the appropriate uses over to
getTypePtrOrNull(). 

A future optimization opportunity would be to distinguish between
cast/dyn_cast and cast_or_null/dyn_cast_or_null; for the former, we
could use getTypePtr() rather than getTypePtrOrNull(), to take another
branch out of the cast/dyn_cast implementation.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121489 91177308-0d34-0410-b5e6-96231b3b80d8

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