Rename getResultType() on function and method declarations to getReturnType()

A return type is the declared or deduced part of the function type specified in
the declaration.

A result type is the (potentially adjusted) type of the value of an expression
that calls the function.

Rule of thumb:

  * Declarations have return types and parameters.
  * Expressions have result types and arguments.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200082 91177308-0d34-0410-b5e6-96231b3b80d8