Remove CHECK and CHECK-NOT from output piped to FileCheck.

The AST node printer removed comments from WebCL Validator
output. Since the rewritten source files preserve comments, we need to
remove FileCheck directive comments from its input stream. Otherwise
FileCheck may match its own directives instead of actual source code.

Signed-off-by: Rami Ylimäki <rami.ylimaki@vincit.fi>