Add .gitignore file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135440 91177308-0d34-0410-b5e6-96231b3b80d8

.gitignore

+#==============================================================================#
+# This file specifies intentionally untracked files that git should ignore.
+# See: http://www.kernel.org/pub/software/scm/git/docs/gitignore.html
+#
+# This file is intentionally different from the output of `git svn show-ignore`,
+# as most of those are useless.
+#==============================================================================#
+
+#==============================================================================#
+# File extensions to be ignored anywhere in the tree.
+#==============================================================================#
+# Temp files created by most text editors.
+*~
+# Merge files created by git.
+*.orig
+# Byte compiled python modules.
+*.pyc
+# vim swap files
+.*.swp
+
+#==============================================================================#
+# Explicit files to ignore (only matches one).
+#==============================================================================#
+cscope.files
+cscope.out