Fix header issues.

Several headers would fail to compile if other headers were not previously
included.  The usual issue is that a class is forward declared, but the
full definition is needed.  The requirement for the definition is use of
isa/dyn_cast or calling functions of pointer-packed data types such as
DenseMap or PointerIntPair.  Add missing includes to these headers.

SVals.h required an out-of-line method definition in the .cpp file to avoid
circular inclusion of headers with BasicValueFactory.h

llvm-svn: 350913