[CodeView] Lower __restrict and other pointer qualifiers correctly

Qualifiers on a pointer or reference type may apply to either the
pointee or the pointer itself. Consider 'const char *' and 'char *
const'. In the first example, the pointee data may not be modified
without casts, and in the second example, the pointer may not be updated
to point to new data.

In the general case, qualifiers are applied to types with LF_MODIFIER
records, which support the usual const and volatile qualifiers as well
as the __unaligned extension qualifier.

However, LF_POINTER records, which are used for pointers, references,
and member pointers, have flags for qualifiers applying to the
*pointer*. In fact, this is the only way to represent the restrict
qualifier, which can only apply to pointers, and cannot qualify regular
data types.

This patch causes LLVM to correctly fold 'const' and 'volatile' pointer
qualifiers into the pointer record, as well as adding support for
'__restrict' qualifiers in the same place.

Based on a patch from Aaron Smith

Differential Revision: https://reviews.llvm.org/D43060

llvm-svn: 326260

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