Bug 1318182 - Support writing Visual Studio 2017 project files; r=glandium

In order for VS2017 to recognize solution and project files as
belonging to VS2017 and not some earlier version, we need to bump
a few versions strings.

The added code path can't be hit yet because MSVS_VERSION cannot
be "2017" until configure detects VS2017. I hacked up MSVS_VERSION
locally and verified that solutions produced with the new code load
in VS2017.

FWIW, the solution loads significantly faster in VS2017 compared to
VS2015!

MozReview-Commit-ID: 2vYAgd6wOsG

--HG--
extra : rebase_source : a29b956bb6eef9071cb46d30460a42ef17658026

python/mozbuild/mozbuild/backend/visualstudio.py

@@ -38,13 +38,17 @@ def get_id(name):
     return str(uuid.uuid5(uuid.NAMESPACE_URL, name)).upper()
 
 def visual_studio_product_to_solution_version(version):
-    if version == '2015':
+    if version == '2017':
+        return '12.00', '15'
+    elif version == '2015':
         return '12.00', '14'
     else:
         raise Exception('Unknown version seen: %s' % version)
 
 def visual_studio_product_to_platform_toolset_version(version):
-    if version == '2015':
+    if version == '2017':
+        return 'v141'
+    elif version == '2015':
         return 'v140'
     else:
         raise Exception('Unknown version seen: %s' % version)