[clang][dataflow] Fix incorrect CXXThisExpr pointee for lambdas

When constructing the `Environment`, the `this` pointee is established
for a `CXXMethodDecl` by looking at its parent. However, inside of
lambdas, a `CXXThisExpr` refers to the captured `this` coming from the
enclosing member function.

When establishing the `this` pointee for a function, we check whether
the function is a lambda, and check for an enclosing member function
to establish the `this` pointee storage location.

Differential Revision: https://reviews.llvm.org/D126413