Fix header issues.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@350913 91177308-0d34-0410-b5e6-96231b3b80d8