[lldb] Return StringRef from PluginInterface::GetPluginName

There is no reason why this function should be returning a ConstString.

While modifying these files, I also fixed several instances where
GetPluginName and GetPluginNameStatic were returning different strings.

I am not changing the return type of GetPluginNameStatic in this patch, as that
would necessitate additional changes, and this patch is big enough as it is.

Differential Revision: https://reviews.llvm.org/D111877

lldb/source/Plugins/ObjectFile/PECOFF/ObjectFilePECOFF.cpp

@@ -1205,6 +1205,3 @@ ObjectFile::Type ObjectFilePECOFF::CalculateType() {
 }
 
 ObjectFile::Strata ObjectFilePECOFF::CalculateStrata() { return eStrataUser; }
-
-// PluginInterface protocol
-ConstString ObjectFilePECOFF::GetPluginName() { return GetPluginNameStatic(); }