Fix typos in clang

Found via codespell -q 3 -I ../clang-whitelist.txt
Where whitelist consists of:

  archtype
  cas
  classs
  checkk
  compres
  definit
  frome
  iff
  inteval
  ith
  lod
  methode
  nd
  optin
  ot
  pres
  statics
  te
  thru

Patch by luzpaz! (This is a subset of D44188 that applies cleanly with a few
files that have dubious fixes reverted.)

Differential revision: https://reviews.llvm.org/D44188

llvm-svn: 329399
diff --git a/clang/docs/doxygen.cfg.in b/clang/docs/doxygen.cfg.in
index 08f4253..0e26769 100644
--- a/clang/docs/doxygen.cfg.in
+++ b/clang/docs/doxygen.cfg.in
@@ -1065,7 +1065,7 @@
 # defined cascading style sheet that is included after the standard style sheets
 # created by doxygen. Using this option one can overrule certain style aspects.
 # This is preferred over using HTML_STYLESHEET since it does not replace the
-# standard style sheet and is therefor more robust against future updates.
+# standard style sheet and is therefore more robust against future updates.
 # Doxygen will copy the style sheet file to the output directory. For an example
 # see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.