swift234

[cmake] Instead of setting LLVM_TABLEGEN_EXE, use our own SWIFT_TABLEGEN_EXE variable.

This eliminates us setting an LLVM_ prefixed variable in a situation where there is
a viable alternative.

We want to eliminate as many of these as possible since they can cause conflicts
with variables set in LLVMConfig.cmake.

I also added some convenience tablegen functions in AddSwiftTableGen.cmake and
changed all tblgen users to use that instead.

rdar://26154980