[analyzer] Support kfree in MallocChecker

Summary:
kmalloc is freed with kfree in the linux kernel. kmalloc support was
added in r204832, but kfree was not. Adding kfree fixes incorrectly
detected memory leaks.

Reviewers: NoQ, nickdesaulniers, dcoughlin, Szelethus

Reviewed By: NoQ, Szelethus

Subscribers: xazax.hun, baloghadamsoftware, szepet, a.sidorin, mikhail.ramalho, Szelethus, donat.nagy, dkrupp, Charusso, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64030

llvm-svn: 364875