Add explanatory comments for the different SRCS_xxx variables used in

the Makefiles for libllvm and libclang.  While here, cleanup a commented
out SRCS entry in libllvmminimal's Makefile.

MFC after:	3 days

lib/clang/libclang/Makefile

@@ -20,6 +20,12 @@ CXXFLAGS.Module.cpp+= -fpermissive
 
 SRCDIR=		tools/clang/lib
 
+# Explanation of different SRCS variants below:
+# SRCS_MIN:	always required, even for bootstrap
+# SRCS_EXT:	required for MK_CLANG_EXTRAS
+# SRCS_FUL:	required for MK_CLANG_FULL
+# SRCS_LDB:	required for MK_LLDB
+
 SRCS_FUL+=	ARCMigrate/ARCMT.cpp
 SRCS_FUL+=	ARCMigrate/ARCMTActions.cpp
 SRCS_FUL+=	ARCMigrate/FileRemapper.cpp

lib/clang/libllvm/Makefile

@@ -26,6 +26,17 @@ CFLAGS+=	-I${LLVM_SRCS}/lib/Target/${arch}
 
 SRCDIR=		lib
 
+# Explanation of different SRCS variants below:
+# SRCS_MIN:	always required, even for bootstrap
+# SRCS_MIW:	required for world stage (after cross-tools)
+# SRCS_EXT:	required for MK_CLANG_EXTRAS
+# SRCS_EXL:	required for MK_CLANG_EXTRAS and MK_LLD
+# SRCS_FUL:	required for MK_CLANG_FULL
+# SRCS_LLD:	required for MK_LLD
+# SRCS_XDB:	required for MK_CLANG_EXTRAS and MK_LLDB
+# SRCS_XDL:	required for MK_CLANG_EXTRAS, MK_LLD and MK_LLDB
+# SRCS_XDW:	required for MK_CLANG_EXTRAS and MK_LLDB in world stage
+
 SRCS_MIN+=	Analysis/AliasAnalysis.cpp
 SRCS_MIN+=	Analysis/AliasAnalysisEvaluator.cpp
 SRCS_MIN+=	Analysis/AliasAnalysisSummary.cpp

lib/clang/libllvmminimal/Makefile

@@ -10,7 +10,6 @@ SRCS+=		Demangle/ItaniumDemangle.cpp
 SRCS+=		Support/APFloat.cpp
 SRCS+=		Support/APInt.cpp
 SRCS+=		Support/ARMTargetParser.cpp
-#SRCS+=		Support/Atomic.cpp
 SRCS+=		Support/CodeGenCoverage.cpp
 SRCS+=		Support/CommandLine.cpp
 SRCS+=		Support/ConvertUTF.cpp