[lld-link] Add comment explaining that /FIXED behavior is correct despite contradicting MSDN.

[thakis]

Summary

Also add a test for /FIXED.

[ruiu]

LGTM

[thakis]

r328879

COFF/Driver.cpp

void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   // Handle /dynamicbase and /fixed. We can't use hasFlag for /dynamicbase
   // because we need to explicitly check whether that option or its inverse was
   // present in the argument list in order to handle /fixed.
   auto *DynamicBaseArg = Args.getLastArg(OPT_dynamicbase, OPT_dynamicbase_no);
   if (DynamicBaseArg &&
       DynamicBaseArg->getOption().getID() == OPT_dynamicbase_no)
     Config->DynamicBase = false;
 
+  // MSDN claims "/FIXED:NO is the default setting for a DLL, and /FIXED is the
+  // default setting for any other project type.", but link.exe defaults to
+  // /FIXED:NO for exe outputs as well. Match behavior, not docs.
   bool Fixed = Args.hasFlag(OPT_fixed, OPT_fixed_no, false);
   if (Fixed) {
     if (DynamicBaseArg &&
         DynamicBaseArg->getOption().getID() == OPT_dynamicbase) {
       error("/fixed must not be specified with /dynamicbase");
     } else {
       Config->Relocatable = false;
       Config->DynamicBase = false;

test/COFF/fixed.test

+# REQUIRES: x86
+# RUN: yaml2obj < %p/Inputs/hello32.yaml > %t.obj
+#
+# RUN: lld-link %t.obj /fixed %p/Inputs/std32.lib /subsystem:console \
+# RUN:   /entry:main@0 /debug /out:%t.fixed.exe
+# RUN: llvm-readobj -file-headers %t.fixed.exe | \
+# RUN:   FileCheck -check-prefix=EXEFIXED %s
+#
+# RUN: lld-link %t.obj %p/Inputs/std32.lib /subsystem:console \
+# RUN:   /entry:main@0 /debug /out:%t.exe
+# RUN: llvm-readobj -file-headers %t.exe | FileCheck -check-prefix=EXEREL %s
+#
+# RUN: yaml2obj < %p/Inputs/export.yaml > %t.obj
+#
+# RUN: lld-link %t.obj /dll /fixed /debug /out:%t.fixed.dll
+# RUN: llvm-readobj -file-headers %t.fixed.dll | FileCheck -check-prefix=DLLFIXED %s
+#
+# RUN: lld-link %t.obj /dll /debug /out:%t.dll
+# RUN: llvm-readobj -file-headers %t.dll | FileCheck -check-prefix=DLLREL %s
+
+EXEFIXED-NOT: IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE
+DLLFIXED-NOT: IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE
+EXEREL: IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE
+DLLREL: IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE