Fix gcc compilation warning in an assert [NFC]

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");
                                      ~

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@360333 91177308-0d34-0410-b5e6-96231b3b80d8