Enable CREATE_SUBDIRS for a performance win, because there are a lot of files being generated.

llvm-svn: 142112
diff --git a/llvm/docs/doxygen.cfg.in b/llvm/docs/doxygen.cfg.in
index 20de077..bc4ab9f 100644
--- a/llvm/docs/doxygen.cfg.in
+++ b/llvm/docs/doxygen.cfg.in
@@ -47,7 +47,7 @@
 # source files, where putting all generated files in the same directory would
 # otherwise cause performance problems for the file system.
 
-CREATE_SUBDIRS         = NO
+CREATE_SUBDIRS         = YES
 
 # The OUTPUT_LANGUAGE tag is used to specify the language in which all
 # documentation generated by doxygen is written. Doxygen will use this