[DebugInfo] Remove LexicalScopes::isCurrentFunctionScope and CSE a use of LexicalScopes::getCurrentFunctionScope

Now that we're sure the only root (non-abstract) scope is the current
function scope, there's no need for isCurrentFunctionScope, the property
can be tested directly instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220451 91177308-0d34-0410-b5e6-96231b3b80d8
[DebugInfo] Add DwarfDebug& to DwarfFile.

Use the DwarfDebug in one function that previously took it as a
parameter, and lay the foundation for use this for other operations
coming soon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220452 91177308-0d34-0410-b5e6-96231b3b80d8
[DebugInfo] Sink DwarfDebug::addCurrentFnArgument down into DwarfFile.

Variable handling will be sunk into DwarfFile so that abstract variables
and the like can be shared across multiple CUs (to handle cross-CU
inlining, for example).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220453 91177308-0d34-0410-b5e6-96231b3b80d8