Fix parsing nested __if_exists blocks

Rather than having kw___if_exists be a special case of
ParseCompoundStatementBody, we can look for kw___if_exists in the big
switch over for valid statement tokens in ParseStatementOrDeclaration.

Nested __if_exists blocks are used in the DECLARE_REGISTRY_RESOURCEID
macro from atlcom.h.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211654 91177308-0d34-0410-b5e6-96231b3b80d8