See rational here: https://reviews.llvm.org/D76173#1922916
Time to compile Attr.h in isolation goes from 2.6s to 1.8s.
Original patch by Johannes, plus some additions from Reid to fix some
clang tooling targets.
Effect on transitive includes is marginal, though:
$ diff -u <(sort thedeps-before.txt) <(sort thedeps-after.txt) \
| grep '^[-+] ' | sort | uniq -c | sort -nr
104 - /usr/local/google/home/rnk/llvm-project/clang/include/clang/AST/OpenMPClause.h
87 - /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Frontend/OpenMP/OMPContext.h
19 - /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/ADT/SmallSet.h
19 - /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/ADT/SetVector.h
14 - /usr/include/c++/9/set
...
Differential Revision: https://reviews.llvm.org/D76184
ba1ffd25c1