Rename baseOf to avoid overloading the term

[deelawn]

Currently the term base has two meanings:

1. the baseOf function returns the base of a declared type
2. pointers have a Base field that returns to the container of the value they point to (struct, array, block)

Look at the comment here and rename baseOf to something like underlyingOf or underlyingTypeOf to differentiate between the two different scenarios and make the code easier for others to understand.

... This is all made additionally confusing by the overloading of the term "base".
baseOf() is only about getting the underlying type for a DeclaredType,
while PointerValues also have a "base" which is the base object within which the pointer points to.

The solution to this overloading is to rename baseOf() and similar references to base
(when referring to the underlying type of a declared type) to underlyingOf(),
while keeping PointerValue.base as base. ...

#1606 (comment)