[clang-cl] Accept the "legacy" -target flag spelling

we already accept "--target=". No reason to not accept "-target" too
(that's the one I typically use for some reason).

Differential revision: https://reviews.llvm.org/D119446

Author: zmodem

clang/include/clang/Driver/Options.td

@@ -4454,7 +4454,9 @@ def mno_vzeroupper : Flag<["-"], "mno-vzeroupper">, Group<m_x86_Features_Group>;
 // These are legacy user-facing driver-level option spellings. They are always
 // aliases for options that are spelled using the more common Unix / GNU flag
 // style of double-dash and equals-joined flags.
-def target_legacy_spelling : Separate<["-"], "target">, Alias<target>;
+def target_legacy_spelling : Separate<["-"], "target">,
+                             Alias<target>,
+                             Flags<[CoreOption]>;
 
 // Special internal option to handle -Xlinker --no-demangle.
 def Z_Xlinker__no_demangle : Flag<["-"], "Z-Xlinker-no-demangle">,

clang/test/Driver/cl-options.c

@@ -772,4 +772,9 @@
 // FAKEDIR: "-libpath:/foo{{/|\\\\}}Lib{{/|\\\\}}10.0.12345.0{{/|\\\\}}ucrt
 // FAKEDIR: "-libpath:/foo{{/|\\\\}}Lib{{/|\\\\}}10.0.12345.0{{/|\\\\}}um
 
+// Accept both the -target and --target= spellings.
+// RUN: %clang_cl --target=i686-pc-windows-msvc19.14.0 -### -- %s 2>&1 | FileCheck -check-prefix=TARGET %s
+// RUN: %clang_cl -target i686-pc-windows-msvc19.14.0  -### -- %s 2>&1 | FileCheck -check-prefix=TARGET %s
+// TARGET: "-triple" "i686-pc-windows-msvc19.14.0"
+
 void f(void) { }