lld-link should have a better diag on missing main / WinMain, probably by defaulting to the subsystem-specific entry point even if main isn't there

[nico]

Bugzilla Link	38972
Resolution	FIXED
Resolved on	Oct 03, 2018 10:05
Version	unspecified
OS	All
Blocks	#38315

Extended Description

If main is missing in link.exe, it prints:

FAILED: test_process.exe test_process.exe.pdb
cmd /c C:/b/depot_tools/win_tools-2_7_6_bin/python/bin/python.exe ../../build/toolchain/win/tool_wrapper.py delete-file ./test_process.exe.pdb && C:/b/depot_tools/win_tools-2_7_6_bin/python/bin/python.exe ../../build/toolchain/win/tool_wrapper.py link-wrapper environment.x86 False link.exe /nologo /OUT:./test_process.exe /PDB:./test_process.exe.pdb @./test_process.exe.rsp
libcmt.lib(exe_main.obj) : error LNK2019: unresolved external symbol _main referenced in function "int __cdecl __scrt_common_main_seh(void)" (?__scrt_common_main_seh@@yahxz)

lld-link prints:

FAILED: test_process.exe test_process.exe.pdb
ninja -t msvc -e environment.x86 -- ../../third_party/llvm-build/Release+Asserts/bin/lld-link.exe /nologo /OUT:./test_process.exe /PDB:./test_process.exe.pdb @./test_process.exe.rsp
lld-link: error: entry point must be defined

"entry point" is less understandable than "missing _main" for most developers; lld-link should have a better diag.

[nico]

r343698 made lld-link's output similar to link.exe's:

lld-link: error: undefined symbol: _main

referenced by f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:78
libcmt.lib(exe_main.obj):("int __cdecl invoke_main(void)" (?invoke_main@@yahxz))
referenced by f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:283
libcmt.lib(exe_main.obj):("int __cdecl __scrt_common_main_seh(void)" (?__scrt_common_main_seh@@yahxz))

Good enough for now