Revert "[Sema] Make getCurFunction() return null outside function parsing"

Revert "[Sema] Make getCurFunction() return null outside function parsing"

This reverts r326965. It seems to have caused repeating test failures in
clang/test/Sema/diagnose_if.c on some buildbots.

I cannot reproduce the problem, and it's not immediately obvious what
the problem is, so let's revert to green.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@326974 91177308-0d34-0410-b5e6-96231b3b80d8