fix(ustring.h): Address ignored annotation nvcc warnings on explicitl…

…y-defaulted functions (#4291)

We're seeing nvcc compilation warnings like the following:
```
ustring.h(869): warning #20012-D: __device__ annotation is ignored on a function("operator=") that is explicitly defaulted on its first declaration
     __attribute__((host)) __attribute__((device)) 
```

According to nvidia's documentation
(https://docs.nvidia.com/cuda/archive/10.1/cuda-c-programming-guide/index.html#compiler-generated-functions),
functions that are explicitly-defaulted should not have execution space
specifiers like \_\_device\_\_ or \_\_host\_\_. Instead the compiler
will determine the annotations from the functions that invoke it.

This patch goes through the ustring and typedesc headers and removes
OIIO_HOSTDEVICE from all defaulted functions.

Signed-off-by: Chris Hellmuth <chellmuth@gmail.com>

src/include/OpenImageIO/typedesc.h

@@ -164,8 +164,7 @@ struct OIIO_UTIL_API TypeDesc {
     TypeDesc (string_view typestring);
 
     /// Copy constructor.
-    OIIO_HOSTDEVICE constexpr TypeDesc (const TypeDesc &t) noexcept = default;
-
+    constexpr TypeDesc (const TypeDesc &t) noexcept = default;
 
     /// Return the name, for printing and whatnot.  For example,
     /// "float", "int[5]", "normal"

src/include/OpenImageIO/ustring.h

@@ -796,11 +796,10 @@ class OIIO_UTIL_API ustringhash {
     ~ustringhash() noexcept = default;
 
     /// Copy construct a ustringhash from another ustringhash.
-    OIIO_HOSTDEVICE constexpr ustringhash(const ustringhash& str) noexcept
-        = default;
+    constexpr ustringhash(const ustringhash& str) noexcept = default;
 
     /// Move construct a ustringhash from another ustringhash.
-    OIIO_HOSTDEVICE ustringhash(ustringhash&& str) noexcept = default;
+    ustringhash(ustringhash&& str) noexcept = default;
 
     /// Construct from a ustring
     ustringhash(const ustring& str) noexcept
@@ -864,9 +863,8 @@ class OIIO_UTIL_API ustringhash {
     }
 
     /// Assign from a ustringhash
-    OIIO_HOSTDEVICE constexpr ustringhash& operator=(const ustringhash& str)
-        = default;
-    OIIO_HOSTDEVICE ustringhash& operator=(ustringhash&& str) = default;
+    constexpr ustringhash& operator=(const ustringhash& str) = default;
+    ustringhash& operator=(ustringhash&& str)                = default;
 
     /// Assign from a ustring
     ustringhash& operator=(const ustring& str)