Setup gclient and presubmit file.

This CL adds the necessary files to use gclient and the depot_tools with
the SPIRV-Tools directory. This allows doing things like `git cl format`
to format code pre-upload and `git cl presubmit -uf` to run presubmit
checks over the code.

The dependencies are all added to the DEPS file and will be
auto-downloaded. They are all pin'd to specific revisions so everyone
has the same checkout. Clang is included in the checkout so it will be
consistent over usages.

Use clang-format
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
new file mode 100644
index 0000000..2ccee9b
--- /dev/null
+++ b/PRESUBMIT.py
@@ -0,0 +1,49 @@
+# Copyright (c) 2018 The Khronos Group Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Presubmit script for SPIRV-Tools.
+
+See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
+for more details about the presubmit API built into depot_tools.
+"""
+
+LINT_FILTERS = [
+  "-build/include",
+  "-build/include_order",
+  "-build/include_what_you_use",
+  "-build/namespaces",
+  "-readability/braces",
+  "-readability/casting",
+  "-readability/fn_size",
+  "-readability/todo",
+  "-runtime/explicit",
+  "-runtime/references",
+  "-whitespace/blank_line",
+  "-whitespace/ending_newline",
+  "-whitespace/line_length",
+  "-whitespace/newline",
+  "-whitespace/operators",
+  "-whitespace/parens",
+  "-whitespace/semicolon",
+  "-whitespace/todo"
+]
+
+
+def CheckChangeOnUpload(input_api, output_api):
+  results = []
+  results += input_api.canned_checks.CheckPatchFormatted(input_api, output_api)
+  results += input_api.canned_checks.CheckChangeLintsClean(
+      input_api, output_api, None, LINT_FILTERS)
+
+  return results