Fix gcc compilation warning in an assert [NFC]

Without this, gcc (7.4) complains with

../tools/clang/lib/Parse/ParseDecl.cpp:3937:63: error: suggest parentheses around '&&' within '||' [-Werror=parentheses]
     assert(!isAlreadyConsumed || RangeEnd != SourceLocation() &&
                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
                                      "both or neither of isAlreadyConsumed and "
                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                      "RangeEnd needs to be set");
                                      ~

llvm-svn: 360333