Rework {clang,lldb,llvm}-tblgen invocations to add --write-if-changed

This flag ensures that the tblgen tools do not actually touch the
produced .inc file, if there are no changes to the contents. In turn,
this may prevent a number of rebuilds of files that include such .inc
files, saving build time.

While here, ensure that the shell invocations to locate the used tblgen
binary do not show unnecessary error messages.

Reported by: des
MFC after: 1 week