C++: Final IR reshuffle

Moved IR flavors into "implementation", with internal files under "implementation/internal". Made `IRBlockConstruction` just a nested module of `IRConstruction`/`SSAConstruction`, so it gets picked up from the `Construction` parameter of the `IR` module, rather than being picked up just from being in the same directory as `IRBlock`.

cpp/ql/test/library-tests/ir/ir/raw_ir.qlref

@@ -1 +1 @@
-semmle/code/cpp/ir/internal/raw/PrintIR.ql
+semmle/code/cpp/ir/implementation/raw/PrintIR.ql

cpp/ql/test/library-tests/ir/ir/raw_sanity.qlref

@@ -1 +1 @@
-semmle/code/cpp/ir/internal/raw/IRSanity.ql
+semmle/code/cpp/ir/implementation/raw/IRSanity.ql

cpp/ql/test/library-tests/ir/ir/unaliased_ssa_ir.qlref

@@ -1 +1 @@
-semmle/code/cpp/ir/internal/unaliased_ssa/PrintIR.ql
+semmle/code/cpp/ir/implementation/unaliased_ssa/PrintIR.ql

cpp/ql/test/library-tests/ir/ir/unaliased_ssa_sanity.qlref

@@ -1 +1 @@
-semmle/code/cpp/ir/internal/unaliased_ssa/IRSanity.ql
+semmle/code/cpp/ir/implementation/unaliased_ssa/IRSanity.ql