1) Don't propagate precision of built-in function arguments to return type when return type is bool (e.g., isnan).
2) Check an additional path for missing default precision qualification, except allow built-in declarations to pass the check.  

git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@22241 e7fa87d3-cd2b-0410-9028-fcbf551c1848