clang-cl: Accept MSVC 2015's `/execution-charset:utf-8` flag.

Also makes -fexec-charset accept utf-8 case-insensitively.
Like https://reviews.llvm.org/D23807, but for execution-charset.
Also replace a few .lower() comparisons with equals_lower().

https://reviews.llvm.org/D23938

llvm-svn: 279866
diff --git a/clang/test/Driver/cl-options.c b/clang/test/Driver/cl-options.c
index 5f7849a..52b40c7 100644
--- a/clang/test/Driver/cl-options.c
+++ b/clang/test/Driver/cl-options.c
@@ -184,6 +184,10 @@
 // RUN: %clang_cl /source-charset:utf-16 -### -- %s 2>&1 | FileCheck -check-prefix=source-charset-utf-16 %s
 // source-charset-utf-16: invalid value 'utf-16'
 
+// /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"
@@ -289,6 +293,7 @@
 // RUN:    /d2FastFail \
 // RUN:    /d2Zi+ \
 // RUN:    /errorReport:foo \
+// RUN:    /execution-charset:utf-8 \
 // RUN:    /FC \
 // RUN:    /Fdfoo \
 // RUN:    /FS \