clang-cl: Expose -isystem.

Like cl.exe, clang-cl allows adding system include directories via the
INCLUDE env var.  Having a driver flag for this functionality is useful,
so add this too.

(In the future, we probably also want to have a flag alternative to
VCINSTALLDIR as used in MSVCToolChain::getVisualStudioBinaries(), and
a way to override the registry accesses in MSVCToolChain::getWindowsSDKDir()
-- maybe -ivcroot= and -iwinsdkroot=?).



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260990 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Driver/Options.td

@@ -1229,7 +1229,8 @@ def iquote : JoinedOrSeparate<["-"], "iquote">, Group<clang_i_Group>, Flags<[CC1
   HelpText<"Add directory to QUOTE include search path">, MetaVarName<"<directory>">;
 def isysroot : JoinedOrSeparate<["-"], "isysroot">, Group<clang_i_Group>, Flags<[CC1Option]>,
   HelpText<"Set the system root directory (usually /)">, MetaVarName<"<dir>">;
-def isystem : JoinedOrSeparate<["-"], "isystem">, Group<clang_i_Group>, Flags<[CC1Option]>,
+def isystem : JoinedOrSeparate<["-"], "isystem">, Group<clang_i_Group>,
+  Flags<[CC1Option, CoreOption]>,
   HelpText<"Add directory to SYSTEM include search path">, MetaVarName<"<directory>">;
 def iwithprefixbefore : JoinedOrSeparate<["-"], "iwithprefixbefore">, Group<clang_i_Group>,
   HelpText<"Set directory to include search path with prefix">, MetaVarName<"<dir>">,

test/Driver/cl-options.c

@@ -437,6 +437,7 @@
 // RUN:     -fno-ms-compatibility \
 // RUN:     -fms-extensions \
 // RUN:     -fno-ms-extensions \
+// RUN:     -isystem=some/path \
 // RUN:     -mllvm -disable-llvm-optzns \
 // RUN:     -Wunused-variable \
 // RUN:     -fmacro-backtrace-limit=0 \