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.

llvm-svn: 211654