Update to reflect changes in Clang.  Rename getResultType() to getReturnType().

In Clang r200082 getResultType() on function and method declarations was renamed
to getReturnType().

git-svn-id: http://include-what-you-use.googlecode.com/svn/trunk@521 812d1f90-f944-d88a-411a-122bdc93e1ea