It is invalid if the same decoration is applied to the same id multiple
times. This adds a check before adding a decoration that the decoration
is not already in the list. If it is, then the duplicate is not added.
Fixes#3627
Specifically, make GLSL link error messages more specific and output
only information relevant to the error.
Also change type printing to more closely reflect GLSL syntax. This
is the default for link error messages, but must me enabled with the
new option --enhanced-msgs for compilation error messages.
Also with --enhanced-msgs, only emit one error message per source
line.
