Use default member initializers in AMDGPUSubtarget and subclasses. This
is to guard against adding a new feature boolean in AMDGPUSubtarget.h
but forgetting to initialize it to false in AMDGPUSubtarget.cpp.
This was mostly autogenerated by:
clang-tidy -checks=-*,cppcoreguidelines-prefer-member-initializer,modernize-use-default-member-init -header-filter=Subtarget -fix lib/Target/AMDGPU/*Subtarget.cpp
Differential Revision: https://reviews.llvm.org/D123613
1. Splitted out some parts of R600 target to separate modules/headers.
2. Reduced some include lists in headers.
3. Found and fixed issue with override `GCNTargetMachine::getSubtargetImpl()`
and `R600TargetMachine::getSubtargetImpl()` had different return value type
than base class.
4. Minor forward declarations cleanup.
Reviewed By: foad
Differential Revision: https://reviews.llvm.org/D108596
