build: Add Windows port of LLVM LTO config.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index a253ce6766a7ccf9265189179aa69fc2b4745db4..b8329fc93dd5886ecb823c03599d91d0ce0652fe 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -6004,6 +6004,12 @@
         ['LD', '<(make_clang_dir)/bin/lld-link'],
       ],
     }],
+    ['clang==1 and use_lto==1 and OS=="win"', {
+      'make_global_settings': [
+        # llvm-lib is a replacement for lib.exe that understands bitcode.

[Nico, 2015/11/11 05:00:00]

we don't bundle this, do we? how big is it?

[pcc1, 2015/11/11 20:22:10]

Hm, since we do apparently bundle lld-link and lld-link supports the /lib flag
(which causes it to act like llvm-lib) we can just do that instead.

And since it turns out we can pass flags in $AR we might as well also pass
/llvmlibthin which causes lld-link to produce thin archives on Windows.

And since that's useful even when not doing LTO, it seems like a good idea to
use it all the time when use_lld==1.

I've done all that in the new patch set.

+        ['AR', '<(make_clang_dir)/bin/llvm-lib'],
+      ],
+    }],
     ['OS=="android" and clang==0', {
       # Hardcode the compiler names in the Makefile so that
       # it won't depend on the environment at make time.
@@ -6087,6 +6093,14 @@
             ],
           }],
         ],
+        'msvs_settings': {
+          'VCCLCompilerTool': {
+            'AdditionalOptions': [
+              '-Xclang',
+              '-emit-llvm-bc',
+            ],
+          },
+        },
       },
     }],
     # Apply a lower LTO optimization level as the default is too slow.
@@ -6106,6 +6120,13 @@
             },
           }],
         ],
+        'msvs_settings': {
+          'VCLinkerTool': {
+            'AdditionalOptions': [
+              '/opt:lldlto=1',
+            ],
+          },
+        },
       },
     }],
     ['use_lto==1 and clang==1 and target_arch=="arm"', {