Avoid printing to stdout directly in library functions.

Previously GlslangToSpv() reported missing/TBD functionalities
by directly writing to stdout using printf. That could cause
problems to callers of GlslangToSpv(). This patch cleans up
the error reporting logic in GlslangToSpv(), TGlslangToSpvTraverser,
and spv::Builder a little bit to use ostringstream.

Also fixed the usage of GlslangToSpv() in GTest fixtures to
capture warnings/errors reported when translating AST to SPIR-V.

SPIRV/SpvBuilder.h

@@ -53,16 +53,17 @@
 #include "spvIR.h"
 
 #include <algorithm>
-#include <memory>
-#include <stack>
 #include <map>
+#include <memory>
 #include <set>
+#include <sstream>
+#include <stack>
 
 namespace spv {
 
 class Builder {
 public:
-    Builder(unsigned int userNumber);
+    Builder(unsigned int userNumber, std::ostringstream& warnError);
     virtual ~Builder();
 
     static const int maxMatrixSize = 4;
@@ -580,13 +581,16 @@ public:
 
     // Our loop stack.
     std::stack<LoopBlocks> loops;
+
+    // The stream for outputing warnings and errors.
+    std::ostringstream& warningsErrors;
 };  // end Builder class
 
 // Use for non-fatal notes about what's not complete
-void TbdFunctionality(const char*);
+void TbdFunctionality(std::ostringstream&, const char*);
 
 // Use for fatal missing functionality
-void MissingFunctionality(const char*);
+void MissingFunctionality(std::ostringstream&, const char*);
 
 };  // end spv namespace