Fix split init of const variables and update lvalue errors

[mppf]

Resolves #15335
Related to #5068

Besides allowing split-init of const variables, this PR adjusts l-value
checking errors from resolution to give an error about constness instead
of one mentioning the term "lvalue" when that makes sense. This helps
with, but does not completely resolve, issue #5068.

Note that the future
https://github.com/chapel-lang/chapel/tree/master/test/variables/constants
has requested different wording for the lvalue error that talks about
const variables instead. This PR moves in that direction but it does not
completely remove the potential for getting an lvalue error. In
particular, something like the following example

proc returnsIntByValue() { return 1; }  
returnsIntByValue() = 1;

still gives an error about lvalues.

The compiler change amounted to adjusting isLegalLvalueActualArg to
return whether the error is from the argument being a call-expression
temporary or is from the argument being const. Then, used that to avoid
giving constness errors for out intent arguments at this time, because
later parts of resolution might change them to split-init. Constness
errors for out intent will still be detected in late const checking.

Reviewed by @vasslitvinov - thanks!

• full local futures testing