Revert changes that migrate to thread_local.

iOS 8 does not support `thread_local`, which is still in use.
Another approach will have to be found.

This change is a revert of the following changes:

a3845240 - "Simplify PoolAlloc with use of thread_local."
abf92c80 - "Deprecate InitializeDll functions"
33585c87 - "Limit visibility of symbols for internal libraries"

Issue: #2346

OGLCompilersDLL/InitializeDll.h

@@ -38,10 +38,10 @@
 
 namespace glslang {
 
-inline bool InitProcess()   { return true; } // DEPRECATED
-inline bool InitThread()    { return true; } // DEPRECATED
-inline bool DetachThread()  { return true; } // DEPRECATED
-inline bool DetachProcess() { return true; } // DEPRECATED
+bool InitProcess();
+bool InitThread();
+bool DetachThread();  // not called from standalone, perhaps other tools rely on parts of it
+bool DetachProcess(); // not called from standalone, perhaps other tools rely on parts of it
 
 } // end namespace glslang