[analyzer] CallEvent: Add helper methods for obtaining the callee stack frame.

Newly added methods allow reasoning about the stack frame of the call (as
opposed to the stack frame on which the call was made, which was always
available) - obtain the stack frame context, obtain parameter regions - even if
the call is not going to be (or was not) inlined, i.e. even if the analysis
has never actually entered the stack frame.

Differential Revision: https://reviews.llvm.org/D49715

llvm-svn: 338474