[llvm-lib] Ignore /NODEFAULTLIB flag

[pzheng]

Summary

It doesn't look like there is anything llvm-lib needs to handle based on
Microsoft's description of the flag.

https://docs.microsoft.com/en-us/cpp/build/reference/managing-a-library?view=msvc-170

[thakis]

Looks fine to me.

May I ask what motivates all these changes? Are you trying to build a particular project? Is it difficult to not pass link.exe-only flags to lib.exe in that project for some reason?

[pzheng]

In D129032#3628870, @thakis wrote:

Looks fine to me.

May I ask what motivates all these changes? Are you trying to build a particular project? Is it difficult to not pass link.exe-only flags to lib.exe in that project for some reason?

Thanks, @thakis. What motivated these changes are some build errors we encountered when building proprietary projects with clang. We don't have control over the flags passed to llvm-lib, so need a way to work around these flag compatibility issues for now.

llvm/lib/ToolDrivers/llvm-lib/Options.td

 // /?? and -?? must be before /? and -? to not confuse lib/Options.
 def help_q : Flag<["/??", "-??", "/?", "-?"], "">, Alias<help>;
 
 //==============================================================================
 // The flags below do nothing. They are defined only for lib.exe compatibility.
 //==============================================================================
 
 def ltcg : F<"ltcg">;
+def nodefaultlib: P<"nodefaultlib", "">;
+def nodefaultlib_all: F<"nodefaultlib">;
 def nologo : F<"nologo">;
 def subsystem : P<"subsystem", "">;