bug#64212: 29.0.91; \Phi missing in TeX input method

[Roland Winkler]

The TeX input method is missing \Phi that should give GREEK CAPITAL
LETTER PHI.  I am surprised I never stumbled upon this previously.  But
apparently that's what it is (with emacs -Q).

(There is a special treatment of variants of the greek letter phi in
latin-ltx.el that goes beyond my knowledge of how input methods are
implemented.)