VS 2015 Update 1 adds support for 'Clang 3.7 with Microsoft CodeGen'.

This fixes these errors when using Clang 3.7 in VS2015:

compiler\translator\ASTMetadataHLSL.cpp(69,10): error : 'visitLoop' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
compiler\translator\ASTMetadataHLSL.cpp(75,10): error : 'visitSelection' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
compiler\translator\ASTMetadataHLSL.cpp(336,10): error : 'visitLoop' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]

Change-Id: Ia9c3029eb2966132827596b55ad9afce34b6f19c
Reviewed-on: https://chromium-review.googlesource.com/317732
Tryjob-Request: Austin Kinross <aukinros@microsoft.com>
Tested-by: Austin Kinross <aukinros@microsoft.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>