Compare type sizes in druntime with ImportC (#16571)

Types for bindings of C libraries in druntime need to match the
C headers. Small differences can result in hard to debug problems.

This test tries to automatically find types with a wrong size.
This is done by also getting type sizes from C headers using ImportC
and comparing them. Differences between the sizes can have different reasons:
* Bugs in ImportC (e.g. for bitfields) can result in a wrong size
* Type definitions in druntime can be wrong
* Different preprocessor options could be used, like _FILE_OFFSET_BITS
* Size differences can be fine, because some structs contain a member
    for the size or a version

Members of structs and unions with the same name are also compared.
For types with potential problems a comparison of the layout is printed.

The test contains a list of known problems, which are only treated as
warnings and not errors.

druntime/Makefile

@@ -421,7 +421,7 @@ HAS_ADDITIONAL_TESTS:=$(shell test -d test && echo 1)
 ifeq ($(HAS_ADDITIONAL_TESTS),1)
  ADDITIONAL_TESTS:=test/init_fini test/exceptions test/coverage test/profile test/cycles test/allocations test/typeinfo \
  test/aa test/cpuid test/gc test/hash test/lifetime test/shared \
- test/thread test/unittest test/imports test/betterc test/stdcpp test/config test/traits
+ test/thread test/unittest test/imports test/betterc test/stdcpp test/config test/traits test/importc_compare
  ifeq (windows,$(OS))
  ADDITIONAL_TESTS+=test/uuid
  else

druntime/test/importc_compare/Makefile

@@ -0,0 +1,19 @@
+include ../common.mak
+
+TESTS:=importc_compare
+
+.PHONY: all clean
+all: $(addprefix $(ROOT)/,$(addsuffix .done,$(TESTS)))
+
+DFLAGS+=-d
+
+$(ROOT)/%.done: $(ROOT)/%$(DOTEXE)
+ @echo Testing $*
+ $(QUIET)$(TIMELIMIT)$(ROOT)/$* $(RUN_ARGS)
+ @touch $@
+
+$(ROOT)/%$(DOTEXE): $(SRC)/%.d
+ $(QUIET)$(DMD) $(DFLAGS) -of$@ $<
+
+clean:
+ rm -rf $(ROOT)