[OpenCL] Use DW_LANG_OpenCL language tag for OpenCL C

Note regarding C++ for OpenCL:

   When compiling C++ for OpenCL, DW_LANG_C_plus_plus* is emitted.

   There is no DWARF language code defined for C++ for OpenCL as of yet,
   but DWARF issue 210514.1 has been raised to request one.

   In the mean time, continuing to emit DW_LANG_C_plus_plus* for C++ for
   OpenCL allows the potential to distinguish between C++ for OpenCL and
   OpenCL C in !DICompileUnit nodes, whereas using DW_LANG_OpenCL for
   C++ for OpenCL would prevent this.

   This change therefore leaves C++ for OpenCL as-is.

Reviewed By: shchenz, Anastasia

Differential Revision: https://reviews.llvm.org/D104118

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -578,6 +578,9 @@ void CGDebugInfo::CreateCompileUnit() {
       LangTag = llvm::dwarf::DW_LANG_C_plus_plus;
   } else if (LO.ObjC) {
     LangTag = llvm::dwarf::DW_LANG_ObjC;
+  } else if (LO.OpenCL && (!CGM.getCodeGenOpts().DebugStrictDwarf ||
+                           CGM.getCodeGenOpts().DwarfVersion >= 5)) {
+    LangTag = llvm::dwarf::DW_LANG_OpenCL;
   } else if (LO.RenderScript) {
     LangTag = llvm::dwarf::DW_LANG_GOOGLE_RenderScript;
   } else if (LO.C99) {

clang/test/CodeGenOpenCL/debug-info-programming-language.cl

@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -dwarf-version=5 -emit-llvm -triple %itanium_abi_triple %s -o - \
+// RUN:   -x cl -O0 -disable-llvm-passes -debug-info-kind=limited \
+// RUN:   | FileCheck --check-prefix=CHECK-OPENCL %s
+// RUN: %clang_cc1 -dwarf-version=3 -emit-llvm -triple %itanium_abi_triple %s -o - \
+// RUN:   -x cl -O0 -disable-llvm-passes -debug-info-kind=limited \
+// RUN:   | FileCheck --check-prefix=CHECK-OPENCL %s
+// RUN: %clang_cc1 -dwarf-version=3 -gstrict-dwarf -emit-llvm -triple %itanium_abi_triple %s -o - \
+// RUN:   -x cl -O0 -disable-llvm-passes -debug-info-kind=limited \
+// RUN:   | FileCheck --check-prefix=CHECK-C99 %s
+// RUN: %clang_cc1 -dwarf-version=5 -gstrict-dwarf -emit-llvm -triple %itanium_abi_triple %s -o - \
+// RUN:   -x cl -O0 -disable-llvm-passes -debug-info-kind=limited \
+// RUN:   | FileCheck --check-prefix=CHECK-OPENCL %s
+
+kernel void empty() {}
+
+// CHECK-OPENCL: distinct !DICompileUnit(language: DW_LANG_OpenCL,
+// CHECK-C99: distinct !DICompileUnit(language: DW_LANG_C99,