Implement pretty diagnostics when doing on-the-fly vector sizing (for vector component access).

For example, before this commit, the following diagnostics would be emitted...

ocu.c:49:12: error: incompatible types assigning 'float  __attribute__((ocu_vector_type(3)))' to 'float4'
    vec4_2 = vec4.rgb; // shorten
    ~~~~~~ ^ ~~~~~~~~
ocu.c:51:7: error: incompatible types assigning 'float  __attribute__((ocu_vector_type(2)))' to 'float'
    f = vec2.xx; // shorten
    ~ ^ ~~~~~~~

Now, the diagnostics look as you would expect...

ocu.c:49:12: error: incompatible types assigning 'float3' to 'float4'
    vec4_2 = vec4.rgb; // shorten
    ~~~~~~ ^ ~~~~~~~~
ocu.c:51:7: error: incompatible types assigning 'float2' to 'float'
    f = vec2.xx; // shorten
    ~ ^ ~~~~~~~

llvm-svn: 40579