Add Decimal.TryFormat span-based method (#15145)

Add Decimal.TryFormat span-based method (#15145)

* Move decimal formatting to managed code

- Move decimal formatting to shared (decimal itself is still not shared)
- Delete VM's decimal formatting code

* Add Decimal.TryFormat span-based method

* Workaround ProjectN compiler bug

Apply the same attribution that's applied to the code in corert.

* Address PR feedback

* Temporarily undo explicit layout change

Seeing whether it's the cause of these failures on Unix (Ubuntu, CentOS, and OSX):
```
JIT.Directed.coverage.oldtests.lclfldadd_cs_r.lclfldadd_cs_r
JIT.Directed.perffix.primitivevt.mixed1_cs_ro.mixed1_cs_ro
JIT.Methodical.fp.exgen.1000w1d_cs_r.1000w1d_cs_r
JIT.Methodical.fp.exgen.1000w1d_cs_ro.1000w1d_cs_ro
JIT.Methodical.fp.exgen.1000w1d_cs_do.1000w1d_cs_do
JIT.Methodical.fp.exgen.1000w1d_cs_d.1000w1d_cs_d
JIT.Methodical.fp.exgen.10w5d_cs_d.10w5d_cs_d
JIT.Methodical.fp.exgen.10w5d_cs_do.10w5d_cs_do
JIT.Methodical.fp.exgen.10w5d_cs_ro.10w5d_cs_ro
JIT.Methodical.fp.exgen.200w1d-02_cs_do.200w1d-02_cs_do
JIT.Methodical.fp.exgen.10w5d_cs_r.10w5d_cs_r
JIT.Methodical.fp.exgen.200w1d-02_cs_ro.200w1d-02_cs_ro
JIT.Directed.perffix.primitivevt.mixed1_cs_do.mixed1_cs_do
```

Example failure:
```
FAILED   - JIT/Methodical/fp/exgen/200w1d-02_cs_ro/200w1d-02_cs_ro.sh
               BEGIN EXECUTION
               /mnt/j/workspace/dotnet_coreclr/master/checked_ubuntu_tst_prtest/bin/tests/Windows_NT.x64.Checked/Tests/coreoverlay/corerun 200w1d-02_cs_ro.exe

               Unhandled Exception: System.OverflowException: Value was either too large or too small for an Int32.
                  at System.Convert.ThrowInt32OverflowException()
                  at System.Convert.ToInt32(Int64 value)
                  at testout1.Func_0()
                  at testout1.Main()
               ./200w1d-02_cs_ro.sh: line 243: 101339 Aborted                 (core dumped) $_DebuggerFullPath "$CORE_ROOT/corerun" $ExePath $CLRTestExecutionArguments
               Expected: 100
               Actual: 134
               END EXECUTION - FAILED
```