[analyzer] Fix invalidation on C++ const methods with arrow syntax.

Conservative evaluation of a C++ method call would invalidate the object,
as long as the method is not const or the object has mutable fields.

When checking for mutable fields, we need to scan the type of the object on
which the method is called, which may be more specific than the type of the
object on which the method is defined, hence we look up the type from the
this-argument expression.

If arrow syntax or implicit-this syntax is used, this-argument expression
has pointer type, not record type, and lookup accidentally failed for that
reason. Obtain object type correctly.

Differential Revision: https://reviews.llvm.org/D48460

llvm-svn: 335555