Add [[clang::suppress(rule, ...)]] attribute

Summary:
This patch implements parsing of [[clang::suppress(rule, ...)]]
and [[gsl::suppress(rule, ...)]] attributes.

C++ Core Guidelines depend heavily on tool support for
rule enforcement. They also propose a way to suppress
warnings [1] which is by annotating any ancestor in AST
with the C++11 attribute [[gsl::suppress(rule1,...)]].
To have a mechanism to suppress non-C++ Core
Guidelines specific, an additional spelling of [[clang::suppress]]
is defined.

For example, to suppress the warning cppcoreguidelines-slicing,
one could do
```
[[clang::suppress("cppcoreguidelines-slicing")]]
void f() { ... code that does slicing ... }
```
or
```
void g() {
  Derived b;
  [[clang::suppress("cppcoreguidelines-slicing")]]
  Base a{b};
  [[clang::suppress("cppcoreguidelines-slicing")]] {
    doSomething();
    Base a2{b};
  }
}
```

This parsing can then be used by clang-tidy, which includes multiple
C++ Core Guidelines rules, to suppress warnings (see
https://reviews.llvm.org/D24888).
For the exact naming of the rule in the attribute, there
are different possibilities, which will be defined in the
corresponding clang-tidy patch.

Currently, clang-tidy supports suppressing of warnings through "//
NOLINT" comments. There are some advantages that the attribute has:
- Suppressing specific warnings instead of all warnings
- Suppressing warnings in a block (namespace, function, compound
  statement)
- Code formatting may split a statement into multiple lines,
  thus a "// NOLINT" comment may be on the wrong line

I'm looking forward to your comments!

[1] https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#inforce-enforcement

Reviewers: alexfh, aaron.ballman, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D24886

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298880 91177308-0d34-0410-b5e6-96231b3b80d8