updates based on PR feedback

There are two major changes (and a few minor ones which are not listed here...see PR discussion for details)

1. Eigen::half implementations for HIP and CUDA have been merged.
This means that
- `CUDA/Half.h` and `HIP/hcc/Half.h` got merged to a new file `GPU/Half.h`
- `CUDA/PacketMathHalf.h` and `HIP/hcc/PacketMathHalf.h` got merged to a new file `GPU/PacketMathHalf.h`
- `CUDA/TypeCasting.h` and `HIP/hcc/TypeCasting.h` got merged to a new file `GPU/TypeCasting.h`

After this change the `HIP/hcc` directory only contains one file `math_constants.h`. That will go away too once that file becomes a part of the HIP install.

2. new macros EIGEN_GPUCC, EIGEN_GPU_COMPILE_PHASE and EIGEN_HAS_GPU_FP16 have been added and the code has been updated to use them where appropriate.
- `EIGEN_GPUCC` is the same as `(EIGEN_CUDACC || EIGEN_HIPCC)`
- `EIGEN_GPU_DEVICE_COMPILE` is the same as `(EIGEN_CUDA_ARCH || EIGEN_HIP_DEVICE_COMPILE)`
- `EIGEN_HAS_GPU_FP16` is the same as `(EIGEN_HAS_CUDA_FP16 or EIGEN_HAS_HIP_FP16)`

test/half_float.cpp

@@ -9,7 +9,7 @@
 
 #include "main.h"
 
-#include <Eigen/src/Core/arch/CUDA/Half.h>
+#include <Eigen/src/Core/arch/GPU/Half.h>
 
 // Make sure it's possible to forward declare Eigen::half
 namespace Eigen {

test/main.h

@@ -68,9 +68,19 @@
 // are defined here and any not-parenthesized min/max call will cause a
 // compiler error.
 #if !defined(__HIPCC__)
-  // HIP headers include the <thread> header which contains not-parenthesized
-  // calls to "max", triggering the following check and causing the compile to fail
-  // so disabling the following checks for HIP
+  //
+  // HIP header files include the following files
+  //  <thread>
+  //  <regex>
+  //  <unordered_map>
+  // which seem to contain not-parenthesized calls to "max"/"min", triggering the following check and causing the compile to fail
+  //
+  // Including those header files before the following macro definition for "min" / "max", only partially resolves the issue
+  // This is because other HIP header files also define "isnan" / "isinf" / "isfinite" functions, which are needed in other
+  // headers.
+  //
+  // So instead choosing to simply disable this check for HIP
+  //
   #define min(A,B) please_protect_your_min_with_parentheses
   #define max(A,B) please_protect_your_max_with_parentheses
   #define isnan(X) please_protect_your_isnan_with_parentheses