Add a ParseAST overload that takes a Sema object, so that the caller

Add a ParseAST overload that takes a Sema object, so that the caller
can create (and hold on to) the Sema object. Also, move Sema-related
initialization/finalization with its various consumers and external
sources into the Sema constructor and destructor, rather than placing
it in ParseAST.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@110952 91177308-0d34-0410-b5e6-96231b3b80d8