Use math symbols instead of CJK punctuation in TeX input method

This seems more appropriate given the context, and additionally
the CJK symbols often render wider, as a full width CJK glyph,
while the math symbols render narrower.  There was some related
discussion in bug#12948, where an analogous change was made
for \langle and \rangle.
* lisp/leim/quail/latin-ltx.el (latin-ltx--define-rules):
Change rules for \llbracket, \rrbracket and \ldata, \rdata to
generate Unicode code points in the Miscellaneous Mathematical
Symbols-A block (U+27E6, U+27E7 and U+27EA, U+27EB) instead of
the CJK Symbols and Punctuation block (U+301A, U+301B and
U+300A , U+300B).

Copyright-paperwork-exempt: yes

lisp/leim/quail/latin-ltx.el

@@ -787,12 +787,12 @@ system, including many technical ones.  Examples:
  ("\\sqrt" ?√)
  ("\\sqrt[3]" ?∛)
  ("\\sqrt[4]" ?∜)
- ("\\llbracket" ?\〚) 			; stmaryrd
- ("\\rrbracket" ?\〛)
+ ("\\llbracket" ?\⟦) 			; stmaryrd
+ ("\\rrbracket" ?\⟧)
  ;; ("\\lbag" ?\〚) 			; fuzz
  ;; ("\\rbag" ?\〛)
- ("\\ldata" ?\《) 			; fuzz/zed
- ("\\rdata" ?\》)
+ ("\\ldata" ?\⟪) 			; fuzz/zed
+ ("\\rdata" ?\⟫)
  ;; From Karl Eichwalder.
  ("\\glq"  ?‚)
  ("\\grq"  ?‘)