Add __amdflang__ define to identify compiler as AMD Flang (llvm#174)

The current compiler version identifies itself as a Flang compiler:
```
#define __flang__ 1
#define __flang_major__ 20
#define __flang_minor__ 0
#define __flang_patchlevel__ 0
```

This PR adds the following define to the list:
```
#define __amdflang__ 1
```

Author: mjklemm

flang/lib/Frontend/CompilerInvocation.cpp

@@ -1465,6 +1465,7 @@ void CompilerInvocation::setDefaultPredefinitions() {
   auto &fortranOptions = getFortranOpts();
   const auto &frontendOptions = getFrontendOpts();
   // Populate the macro list with version numbers and other predefinitions.
+  fortranOptions.predefinitions.emplace_back("__amdflang__", "1");
   fortranOptions.predefinitions.emplace_back("__flang__", "1");
   fortranOptions.predefinitions.emplace_back("__flang_major__",
                                              FLANG_VERSION_MAJOR_STRING);