[AMDGPU] add support for hostcall buffer pointer as hidden kernel argument

Hostcall is a service that allows a kernel to submit requests to the
host using shared buffers, and block until a response is
received. This will eventually replace the shared buffer currently
used for printf, and repurposes the same hidden kernel argument. This
change introduces a new ValueKind in the HSA metadata to represent the
hostcall buffer.

Differential Revision: https://reviews.llvm.org/D70038
diff --git a/llvm/lib/Support/AMDGPUMetadata.cpp b/llvm/lib/Support/AMDGPUMetadata.cpp
index 5f81022..4ea197a 100644
--- a/llvm/lib/Support/AMDGPUMetadata.cpp
+++ b/llvm/lib/Support/AMDGPUMetadata.cpp
@@ -62,6 +62,7 @@
     YIO.enumCase(EN, "HiddenGlobalOffsetZ", ValueKind::HiddenGlobalOffsetZ);
     YIO.enumCase(EN, "HiddenNone", ValueKind::HiddenNone);
     YIO.enumCase(EN, "HiddenPrintfBuffer", ValueKind::HiddenPrintfBuffer);
+    YIO.enumCase(EN, "HiddenHostcallBuffer", ValueKind::HiddenHostcallBuffer);
     YIO.enumCase(EN, "HiddenDefaultQueue", ValueKind::HiddenDefaultQueue);
     YIO.enumCase(EN, "HiddenCompletionAction",
                  ValueKind::HiddenCompletionAction);