[CoroSplit][DebugInfo][nfc] Use more specialize Map arguments

We use a map of Argument->AllocaInst when mapping Arguments to the
AllocaInst created for them.

Said map is declared from "Value" and called a "DbgPtrCache". This
commit:
* replaces Value to the more specialized Argument class, to reflect the
intent better (i.e. we are _always_ mapping Arguments).
* replaces the name "DbgPtrCache" with the more explicit "ArgToAllocaMap",
as it is not clear reading the code what a "DbgPtr" is.

Differential Revision: https://reviews.llvm.org/D149748