Add a less ambiguous macro for Android version.

[danalbert]

Summary

Android has a handful of API levels relevant to developers described
here: https://developer.android.com/studio/build#module-level.
__ANDROID_API__ is too vague and confuses a lot of people. Introduce
a new macro name that is explicit about which one it represents. Keep
the old name around because code has been using it for a decade.

[jiyong]

This LGTM, but just FYI: for darwin targets, these are ENVIRONMENT_<OSNAME>_VERSION_MIN_REQUIRED

[danalbert]

https://github.com/llvm/llvm-project/commit/0849047860a343d8bcf1f828a82d585e89079943

clang/lib/Basic/Targets/OSTargets.h

void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
     DefineStd(Builder, "linux", Opts);
     Builder.defineMacro("__ELF__");
     if (Triple.isAndroid()) {
       Builder.defineMacro("__ANDROID__", "1");
       unsigned Maj, Min, Rev;
       Triple.getEnvironmentVersion(Maj, Min, Rev);
       this->PlatformName = "android";
       this->PlatformMinVersion = VersionTuple(Maj, Min, Rev);
-      if (Maj)
-        Builder.defineMacro("__ANDROID_API__", Twine(Maj));
+      if (Maj) {
+        Builder.defineMacro("__ANDROID_MIN_SDK_VERSION__", Twine(Maj));
+        // This historical but ambiguous name for the minSdkVersion macro. Keep
+        // defined for compatibility.
+        Builder.defineMacro("__ANDROID_API__", "__ANDROID_MIN_SDK_VERSION__");
+      }
     } else {
         Builder.defineMacro("__gnu_linux__");
     }
     if (Opts.POSIXThreads)
       Builder.defineMacro("_REENTRANT");
     if (Opts.CPlusPlus)
       Builder.defineMacro("_GNU_SOURCE");
     if (this->HasFloat128)

clang/test/Preprocessor/init.c

 //
 // RUN: %clang_cc1 -x c++ -fgnuc-version=4.2.1 -std=c++98 -fno-rtti -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix NORTTI %s
 // NORTTI: #define __GXX_ABI_VERSION {{.*}}
 // NORTTI-NOT:#define __GXX_RTTI
 // NORTTI:#define __STDC__ 1
 //
 // RUN: %clang_cc1 -triple arm-linux-androideabi -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix ANDROID %s
 // ANDROID-NOT:#define __ANDROID_API__
+// ANDROID-NOT:#define __ANDROID_MIN_SDK_VERSION__
 // ANDROID:#define __ANDROID__ 1
 // ANDROID-NOT:#define __gnu_linux__
 //
 // RUN: %clang_cc1 -x c++ -triple i686-linux-android -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix I386-ANDROID-CXX %s
 // I386-ANDROID-CXX:#define __STDCPP_DEFAULT_NEW_ALIGNMENT__ 8U
 //
 // RUN: %clang_cc1 -x c++ -triple x86_64-linux-android -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix X86_64-ANDROID-CXX %s
 // X86_64-ANDROID-CXX:#define __STDCPP_DEFAULT_NEW_ALIGNMENT__ 16UL
 //
 // RUN: %clang_cc1 -triple arm-linux-androideabi20 -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix ANDROID20 %s
-// ANDROID20:#define __ANDROID_API__ 20
+// ANDROID20:#define __ANDROID_API__ __ANDROID_MIN_SDK_VERSION__
+// ANDROID20:#define __ANDROID_MIN_SDK_VERSION__ 20
 // ANDROID20:#define __ANDROID__ 1
 // ANDROID-NOT:#define __gnu_linux__
 //
 // RUN: %clang_cc1 -triple lanai-unknown-unknown -E -dM < /dev/null | FileCheck -match-full-lines -check-prefix LANAI %s
 // LANAI: #define __lanai__ 1
 //
 // RUN: %clang_cc1 -E -dM -ffreestanding -triple=amd64-unknown-openbsd6.1 < /dev/null | FileCheck -match-full-lines -check-prefix OPENBSD %s
 // RUN: %clang_cc1 -E -dM -ffreestanding -triple=aarch64-unknown-openbsd6.1 < /dev/null | FileCheck -match-full-lines -check-prefix OPENBSD %s