PGO: llvm-profdata: tool for merging profiles

Introducing llvm-profdata, a tool for merging profile data generated by
PGO instrumentation in clang.

- The name indicates a file extension of <name>.profdata.  Eventually
  profile data output by clang should be changed to that extension.

- llvm-profdata merges two profiles.  However, the name is more general,
  since it will likely pick up more tasks (such as summarizing a single
  profile).

- llvm-profdata parses the current text-based format, but will be
  updated once we settle on a binary format.

<rdar://problem/15949645>

llvm-svn: 201535

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diff --git a/llvm/docs/CommandGuide/llvm-profdata.rst b/llvm/docs/CommandGuide/llvm-profdata.rst
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+llvm-profdata - work with profile data
+======================================
+
+SYNOPSIS
+--------
+
+:program:`llvm-profdata` [-output=output] file1 file2
+
+DESCRIPTION
+-----------
+
+The experimental :program:`llvm-profdata` tool reads two profile data files
+generated by PGO instrumentation and generates a file with merged data.
+
+The profile data format itself is currently textual.
+
+OPTIONS
+-------
+
+.. option:: -output=output
+
+ This option selects the output filename.  If not specified, output is to
+ stdout.
+
+EXIT STATUS
+-----------
+
+:program:`llvm-profdata` returns 1 if it cannot read input files or there is a
+mismatch between their data.