solenv: add COMPILER_EXTERNAL_TOOL

COMPILER_PLUGIN_TOOL has two effects:

1) It makes LinkTarget aware that no object code will be generated by
the compiler (as the compiler is just a tool, not a real compiler), so
e.g.  it's expected that no dependency output will be generated.

2) It puts some of the clang plugins into rewriter mode.

When using some external compiler tool (like a wrapper around
clang-rename), then 1) is wanted, but not 2), i.e. getting rid of the
dependency generation part is wanted, but none of the rewriter plugins
should be activated. So add COMPILER_EXTERNAL_TOOL that does 1) without
2).

Change-Id: I83c21da5763a62d654f93c574d89573cdc7c2138

solenv/gbuild/LinkTarget.mk

@@ -130,7 +130,7 @@ endif
 
 gb_CObject_get_source = $(1)/$(2).c
 
-ifneq ($(COMPILER_PLUGIN_TOOL),)
+ifneq ($(COMPILER_EXTERNAL_TOOL)$(COMPILER_PLUGIN_TOOL),)
 $(call gb_CObject_get_target,%) : $(call gb_CObject_get_source,$(SRCDIR),%) $(gb_FORCE_COMPILE_ALL_TARGET)
 	$(call gb_CObject__tool_command,$*,$<)
 else
@@ -181,7 +181,7 @@ endif
 endif
 endef
 
-ifneq ($(COMPILER_PLUGIN_TOOL),)
+ifneq ($(COMPILER_EXTERNAL_TOOL)$(COMPILER_PLUGIN_TOOL),)
 $(call gb_CxxObject_get_target,%) : $(call gb_CxxObject_get_source,$(SRCDIR),%) $(gb_FORCE_COMPILE_ALL_TARGET)
 	$(call gb_CxxObject__tool_command,$*,$<)
 else
@@ -315,7 +315,7 @@ endef
 
 gb_ObjCxxObject_get_source = $(1)/$(2).mm
 
-ifneq ($(COMPILER_PLUGIN_TOOL),)
+ifneq ($(COMPILER_EXTERNAL_TOOL)$(COMPILER_PLUGIN_TOOL),)
 $(call gb_ObjCxxObject_get_target,%) : $(call gb_ObjCxxObject_get_source,$(SRCDIR),%) $(gb_FORCE_COMPILE_ALL_TARGET)
 	$(call gb_ObjCxxObject__tool_command,$*,$<)
 else
@@ -342,7 +342,7 @@ endif
 
 gb_ObjCObject_get_source = $(1)/$(2).m
 
-ifneq ($(COMPILER_PLUGIN_TOOL),)
+ifneq ($(COMPILER_EXTERNAL_TOOL)$(COMPILER_PLUGIN_TOOL),)
 $(call gb_ObjCObject_get_target,%) : $(call gb_ObjCObject_get_source,$(SRCDIR),%) $(gb_FORCE_COMPILE_ALL_TARGET)
 	$(call gb_ObjCObject__tool_command,$*,$<)
 else