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/Makefiles/header.makefile

@@ -38,12 +38,19 @@ endif
 CYGWIN:=$(findstring CYGWIN, $(shell uname -s))
 LINUX:=$(findstring Linux, $(shell uname -s))
 DARWIN:=$(findstring Darwin, $(shell uname -s))
-
+ifeq ($(CXX), llvm)
+BUILD_CC ?= $(CLANG_BIN)clang
+BUILD_CXX ?= $(CLANG_BIN)clang++
+BUILD_AS ?= $(CLANG_BIN)clang
+BUILD_AR ?= $(CLANG_BIN)llvm-ar
+BUILD_LD ?= $(CLANG_BIN)llvm-ld
+else
 BUILD_CC ?= gcc
 BUILD_CXX ?= g++
 BUILD_AS ?= gcc
 BUILD_AR ?= ar
 BUILD_LD ?= ld
+endif
 LINKER ?= $(BUILD_CC)
 ifeq ($(HOST_ARCH), IA32)
 ARCH_INCLUDE = -I $(MAKEROOT)/Include/Ia32/
@@ -72,14 +79,25 @@ ifeq ($(DARWIN),Darwin)
 BUILD_CFLAGS = -MD -fshort-wchar -fno-strict-aliasing -Wall -Werror \
 -Wno-deprecated-declarations -Wno-self-assign -Wno-unused-result -nostdlib -g
 else
+ifeq ($(CXX), llvm)
+BUILD_CFLAGS = -MD -fshort-wchar -fno-strict-aliasing -fwrapv \
+-fno-delete-null-pointer-checks -Wall -Werror \
+-Wno-deprecated-declarations -Wno-self-assign \
+-Wno-unused-result -nostdlib -g
+else
 BUILD_CFLAGS = -MD -fshort-wchar -fno-strict-aliasing -fwrapv \
 -fno-delete-null-pointer-checks -Wall -Werror \
 -Wno-deprecated-declarations -Wno-stringop-truncation -Wno-restrict \
 -Wno-unused-result -nostdlib -g
 endif
+endif
+ifeq ($(CXX), llvm)
+BUILD_LFLAGS =
+BUILD_CXXFLAGS = -Wno-deprecated-register -Wno-unused-result
+else
 BUILD_LFLAGS =
 BUILD_CXXFLAGS = -Wno-unused-result
-
+endif
 ifeq ($(HOST_ARCH), IA32)
 #
 # Snow Leopard  is a 32-bit and 64-bit environment. uname -m returns i386, but gcc defaults