Relax the assertion in ASTLocation's ctor: if the decl is not the immediate

Relax the assertion in ASTLocation's ctor: if the decl is not the immediate
parent of the stmt, find the immediate parent for the stmt.

This is because sometimes we cannot get the immediate decl of the stmt when
creating the ASTLocation. We can only get a parent of the stmt.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@76159 91177308-0d34-0410-b5e6-96231b3b80d8