clang-format: Add MacroBlock{Begin,End} options

The MacroBlockBegin and MacroBlockEnd options make matching macro identifiers
behave like '{' and '}', respectively, in terms of indentation.

Mozilla code, for example, uses several macros that begin and end a scope.
Previously, Clang-Format removed the indentation resulting in:

    MACRO_BEGIN(...)
    MACRO_ENTRY(...)
    MACRO_ENTRY(...)
    MACRO_END

Now, using the options

    MacroBlockBegin: "^[A-Z_]+_BEGIN$"
    MacroBlockEnd: "^[A-Z_]+_END$"

will yield the expected result:

    MACRO_BEGIN(...)
      MACRO_ENTRY(...)
      MACRO_ENTRY(...)
    MACRO_END

Differential Revision: http://reviews.llvm.org/D10840

llvm-svn: 241363