Namespace collision between interface blocks and other externals

[martiancatboy]

This might be related to the previous interface block issue at #430 (thanks for the prompt fix by the way). I'm now getting namespace collisions between interface blocks, their field names, and other shader inputs and outputs. Finding a minimal program to reproduce the bug is a bit tricky, since name assignments seem unpredictable, but this program

uniform U {vec4 v;};
out vec4 a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v;

gives me the following output (with --format indented):

uniform U{vec4 U;};
out vec4 v,K,J,I,H,G,F,E,D,C,B,A,L,M,N,O,P,Q,R,S,T,U;

My video driver won't accept it because the name "U" is reused three times at global scope. In another instance, I had it fail because an interface block name (uniform U in my first code listing) matched a shader output, so they appear to share the same namespace too.