COMException crash in pdb2pdb after linking System.Private.CoreLib

With https://github.com/dotnet/runtime/pull/37615, we are introducing some ILLink.Substitutions.xml files that direct the linker to trim branches in IL that will never be called. For example, when building for `x64`, any `if (AdvSimd.IsSupported)` branches can be trimmed away, since ARM intrinsics will never be supported for `x64`.

However, Pdb2Pdb is failing in this PR:

```
System.Private.CoreLib -> F:\workspace\_work\1\s\artifacts\bin\coreclr\Windows_NT.x64.Checked\IL\System.Private.CoreLib.dll
PDB0004: token 0x06001C30: Method containing local variables has no local signature
PDB0004: token 0x06001C30: Method containing local variables has no local signature
PDB0004: token 0x06001C31: Method containing local variables has no local signature
PDB0004: token 0x06001C31: Method containing local variables has no local signature
Error HRESULT E_FAIL has been returned from a call to a COM component.
F:\workspace\_work\1\s\.packages\microsoft.dotnet.arcade.sdk\5.0.0-beta.20280.1\tools\SymStore.targets(70,5): error MSB3073: The command ""F:\workspace\_work\1\s\.packages\microsoft.diasymreader.pdb2pdb\1.1.0-beta2-19575-01\tools\Pdb2Pdb.exe" "F:\workspace\_work\1\s\artifacts\bin\coreclr\Windows_NT.x64.Checked\IL\System.Private.CoreLib.dll" /out "F:\workspace\_work\1\s\artifacts\SymStore\Checked\System.Private.CoreLib\netcoreapp2.1\x64\System.Private.CoreLib.pdb" /srcsvrvar SRC_INDEX=public" exited with code 2. [F:\workspace\_work\1\s\src\coreclr\src\System.Private.CoreLib\System.Private.CoreLib.csproj]
##[error].packages\microsoft.dotnet.arcade.sdk\5.0.0-beta.20280.1\tools\SymStore.targets(70,5): error MSB3073: (NETCORE_ENGINEERING_TELEMETRY=Build) The command ""F:\workspace\_work\1\s\.packages\microsoft.diasymreader.pdb2pdb\1.1.0-beta2-19575-01\tools\Pdb2Pdb.exe" "F:\workspace\_work\1\s\artifacts\bin\coreclr\Windows_NT.x64.Checked\IL\System.Private.CoreLib.dll" /out "F:\workspace\_work\1\s\artifacts\SymStore\Checked\System.Private.CoreLib\netcoreapp2.1\x64\System.Private.CoreLib.pdb" /srcsvrvar SRC_INDEX=public" exited with code 2.
```

Debugging into Pdb2Pdb.exe, I see the first problem is that we are trimming away locals from this method:

https://github.com/dotnet/runtime/blob/bd6cbe3642f51d70839912a6a666e5de747ad581/src/libraries/System.Private.CoreLib/src/System/Numerics/BitOperations.cs#L233-L251

The `if (AdvSimd.Arm64.IsSupported)` branch is getting trimmed, but it looks like the local variables are not getting removed from the portable .pdb. Here is the code after trimming:

```C#
public static int PopCount(uint value)
{
if (Popcnt.IsSupported)
{
return (int)Popcnt.PopCount(value);
}
!AdvSimd.Arm64.IsSupported;
return BitOperations.g__SoftwareFallback|9_0(value);
}
```

That is issue https://github.com/mono/linker/issues/1260.

After that, Pdb2Pdb is trying to work on this method:

https://github.com/dotnet/runtime/blob/bd6cbe3642f51d70839912a6a666e5de747ad581/src/libraries/System.Private.CoreLib/src/System/Globalization/CharUnicodeInfo.cs#L231-L248

But it is throwing a `COMException` when calling `OpenScope`:

```
> Microsoft.DiaSymReader.Native.amd64.dll!SymWriter::OpenScope(unsigned int,unsigned int *) Unknown
[Managed to Native Transition]
Microsoft.DiaSymReader.dll!Microsoft.DiaSymReader.SymUnmanagedWriterImpl.OpenScope(int startOffset) Line 261 C#
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverterPortableToWindows.Convert.__OpenScope|6_6(int startOffset, int endOffset, ref Microsoft.DiaSymReader.Tools.PdbConverterPortableToWindows.<>c__DisplayClass6_0 value, ref Microsoft.DiaSymReader.Tools.PdbConverterPortableToWindows.<>c__DisplayClass6_1 value) Line 295 C#
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverterPortableToWindows.Convert(System.Reflection.PortableExecutable.PEReader peReader, System.Reflection.Metadata.MetadataReader pdbReader, Microsoft.DiaSymReader.SymUnmanagedWriter pdbWriter, Microsoft.DiaSymReader.Tools.PortablePdbConversionOptions options) Line 322 C#
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverter.ConvertPortableToWindows(System.Reflection.PortableExecutable.PEReader peReader, System.Reflection.Metadata.MetadataReader pdbReader, Microsoft.DiaSymReader.SymUnmanagedWriter pdbWriter, Microsoft.DiaSymReader.Tools.PortablePdbConversionOptions options) Line 189 C#
Microsoft.DiaSymReader.Converter.dll!Microsoft.DiaSymReader.Tools.PdbConverter.ConvertPortableToWindows(System.Reflection.PortableExecutable.PEReader peReader, System.Reflection.Metadata.MetadataReader pdbReader, System.IO.Stream targetPdbStream, Microsoft.DiaSymReader.Tools.PortablePdbConversionOptions options) Line 171 C#
Pdb2Pdb.exe!Microsoft.DiaSymReader.Tools.Pdb2Pdb.Convert(Microsoft.DiaSymReader.Tools.Pdb2Pdb.Args args) Line 268 C#
Pdb2Pdb.exe!Microsoft.DiaSymReader.Tools.Pdb2Pdb.Main(string[] args) Line 59 C#
```

From doing a little debugging, it appears that the top scope is getting closed in `Microsoft.DiaSymReader.Native.amd64.dll`, and the native code is returning `E_FAIL` when trying to open the scope again. It seems that Pdb2Pdb is getting in some invalid state right before this.

Here is the `.dll` and `.pdb` from the build that is causing the crash:

[System.Private.CoreLib.zip](https://github.com/dotnet/symreader-converter/files/4754063/System.Private.CoreLib.zip)

Unzip that, and run the command:

```
Pdb2Pdb.exe "C:\path\to\System.Private.CoreLib.dll" /out "C:\temp\System.Private.CoreLib.pdb" /srcsvrvar SRC_INDEX=public
```

cc @tmat