Completing the "length()" intrinsic function implementation

Completing the "length()" intrinsic function implementation

The implementation of the length() function was already added
to TParseContext::addFunctionCallOrMethod() in a previous cl,
but it was still unused because addFunctionCallOrMethod() was
always called with the 3rd argument as nullptr. This cl adds
the missing code required to get the length function working.

Change-Id: I3339ae8b60f5f577caa0a39f88c6ed0c626879b2
Reviewed-on: https://swiftshader-review.googlesource.com/3641
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>