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.