[FrontEnd] Allow overriding the default C/C++ -std via CMake vars

[FrontEnd] Allow overriding the default C/C++ -std via CMake vars

Provide two new CMake cache variables -- CLANG_DEFAULT_STD_C
and CLANG_DEFAULT_STD_CXX -- that can be used to override the default
C/ObjC and C++/ObjC++ standards appropriately. They can be set to one of
the identifiers from LangStandards.def, or left unset (the default) to
respect the current platform default.

This option is mostly intended for compiler vendors that may wish
to adjust the defaults their compilers are using. For example, Gentoo
planned to use it to set clang and gcc to matching standards, so that
we could maintain as much compatibility between different compilers
as possible.

The code relies on explicit identifiers rather than the string aliases
for simplicity. This saves us from the necessity of parsing aliases at
build-time or adding additional processing at runtime. For the latter
case, it also adds trivial value check -- if incorrect value is passed,
the code simply fails to compile through referencing an undefined
constant.

If the variable is used to redefine the default standard, the explicit
value overrides the special case for PS4. It is done this way mostly
following other kinds of variables where 'platform defaults' are
redefined.

Differential Revision: https://reviews.llvm.org/D34365

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@326836 91177308-0d34-0410-b5e6-96231b3b80d8