Make CompoundStmtBitfields::NumStmts not a bit-field

Number of statements in CompoundStmt is kept in a bit-field of the common
part of Stmt. The field has 24 bits for the number. To allocate a new
bit field (as attempted in https://reviews.llvm.org/D123952), this
number must be reduced, maximal number of statements in a compound
statement becomes smaller. It can result in compilation errors of some
programs.

With this change the number of statements is kept in a field of type
'unsigned int' rather than in bit-field. To make room in CompoundStmtBitfields
LBraceLoc is moved to fields of CompoundStmt.

Differential Revision: https://reviews.llvm.org/D125635