Accept not only lld-link but also LLD-LINK, for example.

Filenames are case-insensitive on Windows, so when we dispatch based
on argv0, we need to handle it case-insensitively.

Fixes https://bugs.llvm.org/show_bug.cgi?id=32637.

llvm-svn: 300087

lld/test/COFF/driver-windows.test

@@ -0,0 +1,3 @@
+# REQUIRES: system-windows
+# RUN: not LLD-LINK 2>&1 | FileCheck %s
+CHECK: no input files

lld/tools/lld/lld.cpp

@@ -43,9 +43,9 @@ LLVM_ATTRIBUTE_NORETURN static void die(const Twine &S) {
 
 static Flavor getFlavor(StringRef S) {
   return StringSwitch<Flavor>(S)
-      .Cases("ld", "ld.lld", "gnu", Gnu)
-      .Case("link", WinLink)
-      .Case("darwin", Darwin)
+      .CasesLower("ld", "ld.lld", "gnu", Gnu)
+      .CaseLower("link", WinLink)
+      .CaseLower("darwin", Darwin)
       .Default(Invalid);
 }