swift234

SIL Serialization: fix issues.

Clear up data for basic blocks and local values at beginning of
handling a SILFunction.
Fix a bug in handling of StructElementAddr and add testing cases for
enum, initialize_var and struct_element_addr.
Fix a problem in handling types of a SILArgument.

Turn SIL serialization on by default, add a command line "enable-sil-linking"
to turn on SIL deserialization.

Also for readability, change from Name.ResultVal != ~0U to !Name.isMRV()
in SILParser.


Swift SVN r8508