PP: Non-functional: Only use string <-> atom mapping when needed.

Also, eliminate the 'atom' field of TPpToken.

Parsing a real 300 line shader, through to making the AST, is about 10% faster.

Memory is slightly reduced (< 1%).

The whole google-test suite, inclusive of all testing overhead, SPIR-V generation,
etc., runs 3% faster.

Since this is a code *simplification* that leads to perf. improvement, I'm not
going to invest too much more in measuring the perf. than this. The PP code is
simply now in a better state to see how to further rationalize/improve it.

glslang/MachineIndependent/preprocessor/PpTokens.h

@@ -134,7 +134,6 @@ enum EFixedAtoms {
     // preprocessor "keywords"
 
     PpAtomDefine,
-    PpAtomDefined,
     PpAtomUndef,
 
     PpAtomIf,