Lex: make `clang::Preprocessor::macros` work on MSVC

The order of argument construction is reversed on MS ABI on Windows.
When `macros` was invoked, the `end` call is made prior to `begin`.  In
such a case, the DenseMap (`ModuleMap`) is populated after the `end`
iterator is constructed.  This reversal results in the invalidation of
the end iterator, resulting in a failure at runtime (assertion failure
in `DenseMap<T>::operator!=` that "handles are not in sync!").  Ensure
that the end iterator is constructed after the begin iterator.  This
fixes the use of `macros(bool)`, which symptomized as an assertion
failure in the swift compiler in the clang importer.

llvm-svn: 329866