[Sanitizer] Add methods back() and pop_back() to InternalVector. Patch by Sergey Matveev

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@176478 91177308-0d34-0410-b5e6-96231b3b80d8
[Sanitizer] fix signed-unsigned mismatch in test and use correct order of EXPECT_EQ() args

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@176479 91177308-0d34-0410-b5e6-96231b3b80d8