[mlir][spirv] Support import/export Functions and GlobalVariables

"LinkageAttributes" decoration allow a SPIR-V module to import
external functions and global variables, or export functions or
global variables for other SPIR-V modules to link against and use.

Import/export capability is extremely important when using outside
libraries (e.g., intrinsic libraries).

Added decorations:
- LinkageAttributes

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D148749