Disable string tail merging when linking with lld

String tail merging leads to smaller binaries, but they don't compress
as well, leading to increased mini_installer size.

This reduces mini_installer.exe size by 130 KB and increases chrome.dll
and chrome_child.dll by 72 KB and 154 KB, respectively for 32-bit (64-bit
is in the same ballpark).

(This depends on Clang/LLD being rolled past r332149.)

Bug: 838449
Change-Id: I1653e8d522371edb263f742f55ca1aa27ed5dd62
Reviewed-on: https://chromium-review.googlesource.com/1061733
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559459}

build/config/win/BUILD.gn

@@ -118,6 +118,12 @@ config("compiler") {
       "/FIXED:NO",
     ]
 
+    if (use_lld) {
+      # String tail merging leads to smaller binaries, but they don't compress
+      # as well, leading to increased mini_installer size (crbug.com/838449).
+      ldflags += [ "/OPT:NOLLDTAILMERGE" ]
+    }
+
     # TODO(siggi): Is this of any use anymore?
     # /PROFILE ensures that the PDB file contains FIXUP information (growing the
     # PDB file by about 5%) but does not otherwise alter the output binary. It