The signature of `CheckTemplateArgValues` implements error handling via
the `bool` return type, yet always returned false. The single possible
error case instead used `PrintFatalError,` which exits the program
afterward.
This behavior is undesirable: It prevents any further errors from being
printed and makes TableGen less usable as a library as it crashes the
entire process (e.g. `tblgen-lsp-server`).
This PR therefore fixes the issue by using `Error` instead and returning
true if an error occurred. All callers already perform proper error
handling.
As `llvm-tblgen` exits on error, a test was also added to the LSP to
ensure it exits normally despite the error.
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