Using LLVM compiler set to build BaseTools in Linux

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2842

To use LLVM to build BaseTools, first set the CLANG_BIN environment value,
and add "CXX=llvm" to choose LLVM compiler set when using make command.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>

Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>

Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Yuwei Chen<yuwei.chen@intel.com>

BaseTools/Source/C/VfrCompile/Pccts/antlr/makefile

@@ -164,7 +164,11 @@ PCCTS_H=../h
 #
 #   UNIX  (default)
 #
+ifeq ($(CXX), llvm)
+BUILD_CC?=$(CLANG_BIN)clang
+else
 BUILD_CC?=gcc
+endif
 COPT=-O
 ANTLR=${BIN_DIR}/antlr
 DLG=${BIN_DIR}/dlg