Fix deprecated behaviour in folly/Subprocess.h

Summary:
Future C++ standards and compiler upgrades will eliminate deprecated behaviour. `-Wdeprecated` identifies this behaviour and has found some in this code!

Some examples.

**Dynamic exceptions**
```
error: dynamic exception specifications are deprecated [-Werror,-Wdeprecated-dynamic-exception-spec]
```
`throw(...)` has been deprecated since C++11 and removed in C++17. In most cases we can just use `noexcept` in the rest, we can remove this.

**Implicit copy constructors**
```
error: definition of implicit copy constructor for 'XXX' is deprecated because it has a user-declared destructor [-Werror,-Wdeprecated-copy-with-dtor]
```
If you define a destructor, you need to explicitly define a copy constructor.

**Out-ofline constexpr static**
```
error: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Werror,-Wdeprecated]
```
This can be simplified:
```
class MyClass {
    static constexpr my_const = 3;
};

static constexpr MyClass::my_const; // <- No longer needed!
```

Reviewed By: meyering

Differential Revision: D54158200

fbshipit-source-id: 48d316b567810ddf8547038232eeef97b4054f61

folly/Subprocess.h

@@ -237,7 +237,7 @@ class FOLLY_EXPORT SubprocessError : public std::runtime_error {
 class FOLLY_EXPORT CalledProcessError : public SubprocessError {
  public:
   explicit CalledProcessError(ProcessReturnCode rc);
-  ~CalledProcessError() throw() override = default;
+  ~CalledProcessError() noexcept override = default;
   ProcessReturnCode returnCode() const { return returnCode_; }
 
  private:
@@ -250,7 +250,7 @@ class FOLLY_EXPORT CalledProcessError : public SubprocessError {
 class FOLLY_EXPORT SubprocessSpawnError : public SubprocessError {
  public:
   SubprocessSpawnError(const char* executable, int errCode, int errnoValue);
-  ~SubprocessSpawnError() throw() override = default;
+  ~SubprocessSpawnError() noexcept override = default;
   int errnoValue() const { return errnoValue_; }
 
  private: