This is a reland of a474fd7d

The integer clamp used in this transformation is not available in es100
shaders, and float clamp is used instead.

Original change's description:
> Make SH_CLAMP_INDIRECT_ARRAY_BOUNDS do proper AST transformation
>
> This translator flag adds a clamp to non-literal indices to arrays.  Two
> strategies were provisioned, using the clamp intrinsic or a hand-written
> function.   The latter is ununsed in angle, chromium, firefox and
> webkit, so this change removes this option and uses the clamp intrinsic
> unconditionally.
>
> The clamp itself was added at output generation time with special flags
> set on the index node.  This is changed such that a proper AST
> transformation is done and no-special handling would be necessary.
>
> Bug: angleproject:4361
> Bug: angleproject:4889
> Change-Id: Ieccfd2c1c347563fb5282e9fa66d39304e62f2ca
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2935041
> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

Bug: angleproject:4361
Bug: angleproject:4889
Change-Id: I9397ec7e6bdfb706c2a891b33fd3b2b79e883ccc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2940902
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>