[llvm][AIX] Fix triple OS version on PASE (#163392)

The OS version is added to the triple with the value returned by uname.
However, PASE uses different versioning from AIX, so the uname value
needs to be mapped to AIX first.

Author: jakeegan

llvm/lib/TargetParser/Unix/Host.inc

@@ -59,10 +59,30 @@ static std::string updateTripleOSVersion(std::string TargetTripleString) {
     if (TT.getOS() == Triple::AIX && !TT.getOSMajorVersion()) {
       struct utsname name;
       if (uname(&name) != -1) {
+        std::string release = name.release;
+
+        if (strcmp(name.sysname, "OS400") == 0) {
+          /*
+            PASE uses different versioning system than AIX.
+            The following table shows the currently supported PASE
+            releases and the corresponding AIX release:
+            --------------------------
+              PASE    |    AIX
+            --------------------------
+              V7R4    |    7.2 (TL2)
+            --------------------------
+              V7R5    |    7.2 (TL5)
+            --------------------------
+              V7R6    |    7.3 (TL1)
+            --------------------------
+          */
+          release = (release == "4" || release == "5") ? "2" : "3";
+        }
+
         std::string NewOSName = std::string(Triple::getOSTypeName(Triple::AIX));
         NewOSName += name.version;
         NewOSName += '.';
-        NewOSName += name.release;
+        NewOSName += release;
         NewOSName += ".0.0";
         TT.setOSName(NewOSName);
         return TT.str();