[clang-repl] Names declared in if conditions and for-init statements are local...

[clang-repl] Names declared in if conditions and for-init statements are local to the inner context (#84150)

Make TopLevelStmtDecl a DeclContext so that variables defined in statements
are attached to the TopLevelDeclContext. This fixes redefinition errors
from variables declared in if conditions and for-init statements. These
must be local to the inner context (C++ 3.3.2p4), but they had generated
definitions on global scope instead.

This PR makes the TopLevelStmtDecl looking more like a FunctionDecl and
that's fine because the FunctionDecl is very close in terms of semantics.

Additionally, ActOnForStmt() requires a CompoundScope when processing a
NullStmt body.

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Co-authored-by: Vassil Vassilev <v.g.vassilev@gmail.com>