Adjust initial size in StringMap constructor to guarantee no grow()

Summary:
StringMap ctor accepts an initialize size, but expect it to be
rounded to the next power of 2. The ctor can handle that directly
instead of expecting clients to round it. Also, since the map will
resize itself when 75% full, take this into account an initialize
a larger initial size to avoid any growth.

Reviewers: dblaikie

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18344

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 264385