Make locales (and transitively, std::endl) work reliably with gcc.

libc++ currently relies on undefined initialization order of global
initializers when using gcc:

1. __start_std_streams in iostream.cpp calls locale::id::_init, which assigns
   an id to each locale::facet in an initializer

2. Every facet has a static locale::id id, whose constructor sets the facet's
   id to 0

If 2 runs after 1, it clobbers the facet's assigned consecutive id, causing
exceptions to be thrown when e.g. running code like "cout << endl".

To fix this, let _LIBCPP_CONSTEXPR evaluate to "constexpr" instead of nothing
with gcc.  locale::id's constructor is marked _LIBCPP_CONSTEXPR, which ensures
that it won't get an initializer that could potentially run after the
iostream.cpp initializer. (This remains broken when building with msvc.)

Also switch constexpr-specific code in bitset to use __SIZEOF_SIZE_T__ instead
of __SIZE_WIDTH__, because gcc doesn't define the latter.

Pair-programmed/debugged with Dana Jansens.
Support: add additional comment for ARM EH structure

Replicate the fact that ARM::WinEH::RuntimeFunction purposefully does not merge
functions to accommodate raw data access use cases in tools such as readobj.
Pointed out by Renato during post-commit review.

No functional change.
Support: style/documentation cleanup for format

This is purely a documentation/whitespace cleanup for the format support
functions.

The current style does not duplicate the function/class names in the
documentation; conform to this style.

Additionally, there was a large amount of duplication of comments that added no
real value.  Use block comments for the related sets of functions which are used
for type deduction and parameter container classes.

No functional change.
tools: introduce StreamWriter::printBoolean

Add a helper print method to print a boolean value as "Yes" or "No".  This is
intended to be used by the Windows ARM EH printer.
tools: initial implementation of WoA EH decoding

Add support to llvm-readobj to decode Windows ARM Exception Handling data.  This
uses the previously added datastructures to decode the information into a format
that can be used by tests.  This is a necessary step to add support for emitting
Windows on ARM exception handling information.

A fair amount of formatting inspiration is drawn from the Win64 EH printer as
well as the ARM EHABI printer.  This allows for a reasonably thorough look into
the encoded data.