Linker: Eliminate uncalled functions, because they can be ill-defined.

Fixes issue #610. Also provides a testing option to keep uncalled functions.

glslang/Public/ShaderLang.h

@@ -146,6 +146,7 @@ enum EShMessages {
     EShMsgOnlyPreprocessor = (1 << 5),  // only print out errors produced by the preprocessor
     EShMsgReadHlsl         = (1 << 6),  // use HLSL parsing rules and semantics
     EShMsgCascadingErrors  = (1 << 7),  // get cascading errors; risks error-recovery issues, instead of an early exit
+    EShMsgKeepUncalled     = (1 << 8),  // for testing, don't eliminate uncalled functions
 };
 
 //