Clarify driver/frontend -fms-compatibility help text

Better describe the flag that enables drop-in MSVC compatibility, including
ability to parse MS standard headers. This is intended to distinguish it from
-fms-extensions, the more established and 'gentler' flag also supported by GCC.

The new wording matches up with the internal description introduced in r198936.
Still room for improvement (e.g. C++ is part of the product name, yet the flag
also applies to C) but it's a step forward from "Microsoft mode".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199247 91177308-0d34-0410-b5e6-96231b3b80d8
Darwin: add __sinpi (etc) and __exp10 libbuiltins

These functions have the same constness properties of the normal libm
functions, which allows LLVM to optimise code better in general. There
are also a couple of specific optimisations that only trigger when
these are properly marked.

rdar://problem/13729466

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199249 91177308-0d34-0410-b5e6-96231b3b80d8
Remove the -cxx-abi command-line flag.

This makes the C++ ABI depend entirely on the target: MS ABI for -win32 triples,
Itanium otherwise. It's no longer possible to do weird combinations.

To be able to run a test with a specific ABI without constraining it to a
specific triple, new substitutions are added to lit: %itanium_abi_triple and
%ms_abi_triple can be used to get the current target triple adjusted to the
desired ABI. For example, if the test suite is running with the i686-pc-win32
target, %itanium_abi_triple will expand to i686-pc-mingw32.

Differential Revision: http://llvm-reviews.chandlerc.com/D2545

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199250 91177308-0d34-0410-b5e6-96231b3b80d8