restyle builtin modules to match project style

Now that the IWYU cleanup has been merged compile all, not just a couple, of
the builtin modules independent of builtin.cpp. That is, no longer `#include
builtin_NAME.cpp` in builtin.cpp. This is more consistent, more in line with
what developers expect, and is likely to reduce mistakes.

Reduces lint errors from 384 to 336 (-13%). Line count from 6307 to 4988 (-21%).

Another step in resolving issue #2902.

build_tools/iwyu.linux.imp

@@ -14,4 +14,6 @@
     { symbol: ["size_t", "private", "<stddef.h>", "public"] },
     { symbol: ["size_t", "private", "<stdlib.h>", "public"] },
     { symbol: ["uint64_t", "private", "<sys/types.h>", "public"] },
+    { symbol: ["memset", "private", "<string.h>", "public"] },
+    { symbol: ["strerror", "private", "<string.h>", "public"] },
 ]