[test] Add a .clang-format file for the shell test.

[JDevlieghere]

Summary

The API tests have a .clang-format file that disables formatting altogether. While this is needed for some tests, it also leads to inconsistency between test files. The shell tests suffer from a similar problem: a test with a source-file extension (.c, .cpp) will get formatted, potentially breaking up lines and leading to invalid RUN commands. Rather than completely disabling formatting here, I propose to not enforce a line limit instead. That way tests will be consistent, but you can still have long run commands (as is not uncommon in LLVM either) and use breakpoints with patters that extend beyond 80 cols.

[teemperor]

LGTM, thanks!

lldb/test/Shell/.clang-format

+BasedOnStyle: LLVM
+# We don't want clang-format to introduce line breaks in RUN commands.
+ColumnLimit: 999