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/lib/Basic/DiagnosticIDs.cpp b/clang/lib/Basic/DiagnosticIDs.cpp
index c4c425d..b25057f 100644
--- a/clang/lib/Basic/DiagnosticIDs.cpp
+++ b/clang/lib/Basic/DiagnosticIDs.cpp
@@ -470,7 +470,7 @@
       Result = diag::Severity::Error;
   }
 
-  // If -Wfatal-errors is enabled, map errors to fatal unless explicity
+  // If -Wfatal-errors is enabled, map errors to fatal unless explicitly
   // disabled.
   if (Result == diag::Severity::Error) {
     if (State->ErrorsAsFatal && !Mapping.hasNoErrorAsFatal())