Empty declarations in ESSL shaders currently result in errors on several
platforms. Prune empty declarations that are not struct or interface block
declarations from the AST.

TEST=WebGL conformance tests, angle_unittests, angle_end2end_tests
BUG=angleproject:980

Change-Id: I9e3abf8134e6dfed0253cc83f69ce0ee92b0e0e7
Reviewed-on: https://chromium-review.googlesource.com/266841Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>