Make clang-format indent nested preprocessor directives

[tautschnig]

Since clang-format 6 there is support to indent preprocessor directives,
enabling formatting like

 #if foo
 #  define bar
 #endif

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 98f619f).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/106430068

[martin-cs]

Does this mean we are going to have to reformat all of the includes inside header guards?

[tautschnig]

Does this mean we are going to have to reformat all of the includes inside header guards?

No, clang-format is able to tell apart header guards from other #if. What you've seen in #4458 is just because that entire file is, in addition to the header guard, also #if guarded for Linux-only.