Adjusting code interface

Adjusting code interface

glslang/include/intermediate.h -> Add a new interface to set TIntermBranch's expression.

glslang/include/Types.h -> Add interface to set Type's basicType and add interface to get basicType form a TSampler.

glslang/MachineIndependent/intermediate.cpp -> Part of the code in createConversion been encapsulating as a new function called buildConvertOp

glslang/MachineIndependent/localintermediate.h -> Export createConversion and
buildConvertOp as a public function

glslang/Public/ShaderLang.h -> Add interface to get shader object and shader source.