[build-script] Add buildbot_linux,lld preset

This patch adds a preset that defaults the toolchain to use lld instead
of gold on Linux.

Added presets are
  - `buildbot_linux,lld`
  - `buildbot_linux,lld,no_assertions`
  - `buildbot_linux,lld,no_test`
  - `buildbot_linux,lld,no_assertions,no_test`

Author: etcwilde

utils/build-presets.ini

@@ -1029,6 +1029,31 @@ mixin-preset=buildbot_linux_crosscompile_android,tools=RA,stdlib=RD,build
 
 android-arch=x86_64
 
+[preset: mixin_lld]
+extra-cmake-options=
+  -DSWIFT_USE_LINKER=lld
+  -DCLANG_DEFAULT_LINKER=lld
+
+[preset: buildbot_linux,lld]
+mixin-preset=
+  buildbot_linux
+  mixin_lld
+
+[preset: buildbot_linux,lld,no_assertions]
+mixin-preset=
+  buildbot_linux,no_assertions
+  mixin_lld
+
+[preset: buildbot_linux,lld,no_test]
+mixin-preset=
+  buildbot_linux,no_test
+  mixin_lld
+
+[preset: buildbot_linux,lld,no_assertions,no_test]
+mixin-preset=
+  buildbot_linux,no_assertions,no_test
+  mixin_lld
+
 # Ubuntu 18.04 preset for backwards compat and future customizations.
 [preset: buildbot_linux_1804]
 mixin-preset=buildbot_linux