[llvm] [flang-rt] Optimise ShallowCopy and use it in CopyInAssign (PR #140569)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-aarch64-sharedlibs` running on `linaro-flang-aarch64-sharedlibs` while building `flang-rt` at step 5 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/80/builds/13606

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (build-unified-tree) failure: build (failure)
...
111.886 [122/1/51] Building C object openmp/libompd/gdb-plugin/CMakeFiles/ompdModule.dir/ompdAPITests.c.o
112.015 [121/1/52] Linking C shared module openmp/libompd/gdb-plugin/python-module/ompd/ompdModule.so
112.027 [120/1/53] Generating python-module/ompd/__init__.py
118.672 [119/1/54] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/llvm-project/flang/lib/Decimal/binary-to-decimal.cpp.o
124.236 [118/1/55] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/llvm-project/flang/lib/Decimal/decimal-to-binary.cpp.o
126.390 [117/1/56] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/ISO_Fortran_binding.cpp.o
127.221 [116/1/57] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/allocator-registry.cpp.o
130.196 [115/1/58] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/allocatable.cpp.o
135.043 [114/1/59] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/array-constructor.cpp.o
139.533 [113/1/60] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/assign.cpp.o
FAILED: flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/assign.cpp.o 
/home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/build/./bin/clang++ --target=aarch64-unknown-linux-gnu -D_DEBUG -D_GLIBCXX_ASSERTIONS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/llvm-project/flang-rt/include -I/home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/llvm-project/flang-rt/../flang/include -I/home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/build/runtimes/runtimes-bins/flang-rt -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wno-unnecessary-virtual-specifier -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=gnu++17 -UNDEBUG -fno-lto -fno-exceptions -fno-rtti -funwind-tables -fno-asynchronous-unwind-tables -U_GLIBCXX_ASSERTIONS -U_LIBCPP_ENABLE_ASSERTIONS -MD -MT flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/assign.cpp.o -MF flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/assign.cpp.o.d -o flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/assign.cpp.o -c /home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/llvm-project/flang-rt/lib/runtime/assign.cpp
/home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/llvm-project/flang-rt/lib/runtime/assign.cpp:594:17: error: use of undeclared identifier 'kNoAsyncId'
  594 |   temp.Allocate(kNoAsyncId);
      |                 ^~~~~~~~~~
1 error generated.
ninja: build stopped: subcommand failed.
FAILED: runtimes/runtimes-stamps/runtimes-build /home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/build/runtimes/runtimes-stamps/runtimes-build 
cd /home/tcwg-buildbot/worker/flang-aarch64-sharedlibs/build/runtimes/runtimes-bins && /usr/local/bin/cmake --build .
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/140569