build.ps1: introduce the ability to do LTO

While it is possible to do LTO with MSVC, we cannot do that just yet.
This at least allows some experimentation with LTO with clang-cl.

Author: compnerd

utils/build.ps1

@@ -201,6 +201,8 @@ param
   [string[]] $Test = @(),
 
   [switch] $IncludeDS2 = $false,
+  [ValidateSet("none", "full", "thin")]
+  [string] $LTO = "none",
   [switch] $IncludeNoAsserts = $false,
   [ValidateSet("debug", "release")]
   [string] $FoundationTestConfiguration = "debug",
@@ -2055,6 +2057,10 @@ function Get-CompilersDefines([Hashtable] $Platform, [string] $Variant, [switch]
     $SwiftFlags += @("-Xcc", "-D_ALLOW_COMPILER_AND_STL_VERSION_MISMATCH");
   }
 
+  if ($LTO -ne "none") {
+    $SwiftFlags += @("-use-ld=lld");
+  }
+
   return $TestDefines + $DebugDefines + @{
     CLANG_TABLEGEN = (Join-Path -Path $BuildTools -ChildPath "clang-tblgen.exe");
     CLANG_TIDY_CONFUSABLE_CHARS_GEN = (Join-Path -Path $BuildTools -ChildPath "clang-tidy-confusable-chars-gen.exe");
@@ -2068,6 +2074,17 @@ function Get-CompilersDefines([Hashtable] $Platform, [string] $Variant, [switch]
     LLDB_TEST_MAKE = "$BinaryCache\GnuWin32Make-4.4.1\bin\make.exe";
     LLVM_CONFIG_PATH = (Join-Path -Path $BuildTools -ChildPath "llvm-config.exe");
     LLVM_ENABLE_ASSERTIONS = $(if ($Variant -eq "Asserts") { "YES" } else { "NO" })
+    LLVM_ENABLE_LTO = $(switch ($LTO) {
+      "none" { "OFF" }
+      default {
+        if ($UseHostToolchain) { throw "LTO is not supported with the host toolchain" }
+        "$LTO"
+      }
+    })
+    LLVM_ENABLE_LLD = $(switch ($LTO) {
+      "none" { "NO" }
+      default { "YES" }
+    })
     LLVM_EXTERNAL_SWIFT_SOURCE_DIR = "$SourceCache\swift";
     LLVM_HOST_TRIPLE = $Platform.Triple;
     LLVM_NATIVE_TOOL_DIR = $BuildTools;