[libc++] Fix std::forward for r-value references

The explicit cast is necessary even though the value for `t` is passed in as an r-value, because it becomes an l-value as soon as it is assigned to the variable.
Status Job ID Name Coverage
  Git Update
failed #424024
nix
git-update

 
  Tests
skipped #424026
allowed to fail manual
tests-on-llvm-10
skipped #424027
nix
tests-on-llvm-11
skipped #424025
nix
tests-on-llvm-9
 
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failed
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