[cmake] Add function for building native tool

Instead of duplicating functionality for building native versions of
tblgen and llvm-config, add a function to set up a native tool build.
This will also be used for llvm-nm in a follow-up.

This should be NFC for tblgen, besides the slightly different COMMENT
for the custom command (it'll display the tablegen target name instead
of always saying TableGen).  For the native llvm-config, it's a behavior
change in that we'll use llvm_ExternalProject_BuildCmd instead of
constructing the build command manually, always build in Release, and
reference the correct binary path for multi-config generators. I believe
all of these changes to be bug fixes.

Differential Revision: https://reviews.llvm.org/D60024

llvm-svn: 357486

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