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
MSVC does not create a new scope for the body of an __if_exists compound
statement. Clang already gets this right today, but it was untested.
llvm-svn: 211650