[TargetParser] Fix flaky installs of generated headers (#137853)

The `llvm-headers` target wasn't depending on the generated TargetParser
headers, so they'd be flakily installed or not installed depending on
which order the build steps ran in. Add an explicit dependency to fix
this, and switch to a single `target_parser_gen` target to mirror the
pattern used by `intrinsics_gen` (which also fixes a few other missing
dependencies). Switch `llvm-headers` to use `add_dependencies` instead
of `DEPENDS` for the tablegen dependencies as well, since `DEPENDS` is
only intended for creating a file-level dependency on the output of an
`add_custom_command` in the same CMakeLists.txt (see `DEPENDS` under
https://cmake.org/cmake/help/latest/command/add_custom_target.html).

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