Correct precision qualification on built-in functions.

This is a replacement commit for pull request #238.

This is a design change, followed by implementation change that
A) fixes the changes caused by the design change, and
B) fixes some cases that were originally incorrect.

The design change is to not give built-in functions default precision qualification.
This is to allow the rule that the precision of some built-in functions adopt their
precision qualification from the calling arguments.  This is A above.

A consequence of this design change is that all built-ins that are supposed to have
an explicit precision qualifier must now be declared that way.  So, a lot more
built-in declarations now have precision qualifiers, just to keep things the same.
This is B above.

glslang/MachineIndependent/ParseHelper.h

@@ -334,7 +334,7 @@ protected:
     TParseContext(TParseContext&);
     TParseContext& operator=(TParseContext&);
 
-    bool parsingBuiltins;        // true if parsing built-in symbols/functions
+    const bool parsingBuiltins;        // true if parsing built-in symbols/functions
     static const int maxSamplerIndex = EsdNumDims * (EbtNumTypes * (2 * 2 * 2)); // see computeSamplerTypeIndex()
     TPrecisionQualifier defaultSamplerPrecision[maxSamplerIndex];
     bool afterEOF;