[clang] Fix a warning

This patch fixes:

  clang/lib/AST/Interp/ByteCodeExprGen.cpp:978:24: warning: variable
  ‘T’ set but not used [-Wunused-but-set-variable]

T and ReturnType were introduced on August 19, 2022 in commit
8e41e6a4.

Their last uses were removed on October 13, 2022 in commit
0e754cfa.