Add PragmaHandler for MSVC pragma execution_character_set

__pragma(execution_character_set(push, "UTF-8")) is used in
TraceLoggingProvider.h. This commit implements a no-op handler for
compatability, similar to how the flag -fexec_charset is handled.

Patch by Matt Gardner!

Differential Revision: https://reviews.llvm.org/D58530

llvm-svn: 356185