Memory/Perf: For link-mode, isolate file I/O so API can be looped over.

Separating file I/O from compile/link lets the compile/link be done
repeatedly in a loop for testing and measuring of performance and
memory footprint, including seeing memory growth over time for
functional-level memory-leak testing.

While the older compile-only mode already had this functionality,
and typically showed no memory leaks, SPIR-V uses the link path,
has pending "TODO" for memory freeing, and this shows several
kilobytes of leaking per compile-link. Most likely, pending
merge request 131 will address much of this.

glslang/OSDependent/Windows/ossource.cpp

@@ -147,6 +147,8 @@ void OS_Sleep(int milliseconds)
     Sleep(milliseconds);
 }
 
+//#define DUMP_COUNTERS
+
 void OS_DumpMemoryCounters()
 {
 #ifdef DUMP_COUNTERS