[llvm-cov] Multi-threaded implementation of prepareFileReports method.

Summary:
Local testing has demonstrated a great speed improvement, compare the following:

1) Existing version:
```
$ time llvm

[llvm-cov] Multi-threaded implementation of prepareFileReports method.

Summary:
Local testing has demonstrated a great speed improvement, compare the following:

1) Existing version:
```
$ time llvm-cov show -format=html -output-dir=report -instr-profile=... ...
The tool has been launched: 00:00:00
Loading coverage data: 00:00:00
Get unique source files: 00:00:33
Creating an index out of the source files: 00:00:34
Going into prepareFileReports: 00:00:34
Going to emit summary information for each file: 00:28:55 <-- 28:21 min!
Going to emit links to files with no function: 00:28:55
Launching 32 threads for generating HTML files: 00:28:55

real 37m43.651s
user 112m5.540s
sys 7m39.872s
```

2) Multi-threaded version with 32 CPUs:
```
$ time llvm-cov show -format=html -output-dir=report -instr-profile=... ...
The tool has been launched: 00:00:00
Loading coverage data: 00:00:00
Get unique source files: 00:00:38
Creating an index out of the source files: 00:00:40
Going into prepareFileReports: 00:00:40
Preparing file reports using 32 threads: 00:00:40
# Creating thread tasks for the following number of files: 16422
Going to emit summary information for each file: 00:01:57 <-- 1:17 min!
Going to emit links to files with no function: 00:01:58
Launching 32 threads for generating HTML files: 00:01:58

real 11m2.044s
user 134m48.124s
sys 7m53.388s
```

Reviewers: vsk, morehouse

Reviewed By: vsk

Subscribers: Dor1s, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D41206

llvm-svn: 321871

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