clang-cl: Support MSVC2015's /validate-charset flag.

[thakis]

Summary

Clang always assumes that files are utf-8. If an invalidly encoded character is used in an identifier, clang always errors. If it's used in a character literal, clang warns Winvalid-source-encoding (on by default). Clang never checks the encoding of things in comments (adding this seems like a nice feature if it doesn't impact performance).

For cl.exe /utf-8 (which enables /validate-charset), if a bad character is used in an identifier, it emits both an error and a warning. If it's used in a literal or a comment, it emits a warning.

So mapping /validate-charset to -Winvalid-source-encoding seems like a fairly decent fit.

[hans]

lgtm

[thakis]

r279872, thanks!

include/clang/Driver/CLCompatOptions.td

 def _SLASH_source_charset : CLCompileJoined<"source-charset:">,
   HelpText<"Source encoding, supports only UTF-8">, Alias<finput_charset_EQ>;
 def _SLASH_execution_charset : CLCompileJoined<"execution-charset:">,
   HelpText<"Runtime encoding, supports only UTF-8">, Alias<fexec_charset_EQ>;
 def _SLASH_std : CLCompileJoined<"std:">,
   HelpText<"Language standard to compile for">;
 def _SLASH_U : CLJoinedOrSeparate<"U">, HelpText<"Undefine macro">,
   MetaVarName<"<macro>">, Alias<U>;
+def _SLASH_validate_charset : CLFlag<"validate-charset">,
+  Alias<W_Joined>, AliasArgs<["invalid-source-encoding"]>;
+def _SLASH_validate_charset_ : CLFlag<"validate-charset-">,
+  Alias<W_Joined>, AliasArgs<["no-invalid-source-encoding"]>;
 def _SLASH_W0 : CLFlag<"W0">, HelpText<"Disable all warnings">, Alias<w>;
 def _SLASH_W1 : CLFlag<"W1">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W2 : CLFlag<"W2">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W3 : CLFlag<"W3">, HelpText<"Enable -Wall">, Alias<Wall>;
 def _SLASH_W4 : CLFlag<"W4">, HelpText<"Enable -Wall and -Wextra">, Alias<WCL4>;
 def _SLASH_Wall : CLFlag<"Wall">, HelpText<"Enable -Wall and -Wextra">, Alias<WCL4>;
 def _SLASH_WX : CLFlag<"WX">, HelpText<"Treat warnings as errors">,
   Alias<W_Joined>, AliasArgs<["error"]>;

test/Driver/cl-options.c

 // /execution-charset: should warn on everything except UTF-8.
 // RUN: %clang_cl /execution-charset:utf-16 -### -- %s 2>&1 | FileCheck -check-prefix=execution-charset-utf-16 %s
 // execution-charset-utf-16: invalid value 'utf-16'
 //
 // RUN: %clang_cl /Umymacro -### -- %s 2>&1 | FileCheck -check-prefix=U %s
 // RUN: %clang_cl /U mymacro -### -- %s 2>&1 | FileCheck -check-prefix=U %s
 // U: "-U" "mymacro"
 
+// RUN: %clang_cl /validate-charset -### -- %s 2>&1 | FileCheck -check-prefix=validate-charset %s
+// validate-charset: -Winvalid-source-encoding
+
+// RUN: %clang_cl /validate-charset- -### -- %s 2>&1 | FileCheck -check-prefix=validate-charset_ %s
+// validate-charset_: -Wno-invalid-source-encoding
+
 // RUN: %clang_cl /vd2 -### -- %s 2>&1 | FileCheck -check-prefix=VD2 %s
 // VD2: -vtordisp-mode=2
 
 // RUN: %clang_cl /vmg -### -- %s 2>&1 | FileCheck -check-prefix=VMG %s
 // VMG: "-fms-memptr-rep=virtual"
 
 // RUN: %clang_cl /vmg /vms -### -- %s 2>&1 | FileCheck -check-prefix=VMS %s
 // VMS: "-fms-memptr-rep=single"