[.gitignore] ignore vim swap files harder

This matches the patterns for vim swap files in llvm/.gitignore

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239875 91177308-0d34-0410-b5e6-96231b3b80d8

.gitignore

@@ -16,7 +16,7 @@
 # Byte compiled python modules.
 *.pyc
 # vim swap files
-.*.swp
+.*.sw?
 .sw?
 
 #==============================================================================#