__thread: Move constant init check to CheckCompleteVariableDeclaration

We were emitting dynamic initializers for __thread variables if there
was no explicit initializer, as in this test case:

struct S { S(); };
__thread S s;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207675 91177308-0d34-0410-b5e6-96231b3b80d8